docs: Prepare docs for fileupload editor (#1524)

This PR should be merged once the feature is in Apify Console.

### Related PRs
 - apify/apify-shared-js#509

---------

Co-authored-by: Michał Olender <[email protected]>

Author: valekjo

sources/platform/actors/development/actor_definition/input_schema/images/input-schema-fileupload-input.png

@@ -223,11 +223,19 @@ The `anyDate` property renders a date picker that accepts both absolute and rela
 
 ![Apify Actor input schema - country input](./images/input-schema-date-both.png)
 
+The `fileupload` editor enables users to specify a file as input. The input is passed to the Actor as a string. It is the Actor author's responsibility to interpret this string, including validating its existence and format.
+
+![Apify Actor input schema - fileupload input](./images/input-schema-fileupload-input.png)
+
+The user provides either a URL or uploads the file to a key-value store (existing or new).
+
+![Apify Actor input schema - fileupload input options](./images/input-schema-fileupload-modal.png)
+
 Properties:
 
 | Property     | Value                                                                                                                                                | Required                                   | Description                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
 |--------------|------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
-| `editor`     | One of <ul><li>`textfield`</li><li>`textarea`</li><li>`javascript`</li><li>`python`</li><li>`select`</li><li>`datepicker`</li><li>`hidden`</li></ul> | Yes                                        | Visual editor used for <br/>the input field.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
+| `editor`     | One of <ul><li>`textfield`</li><li>`textarea`</li><li>`javascript`</li><li>`python`</li><li>`select`</li><li>`datepicker`</li><li>`fileupload`</li><li>`hidden`</li></ul> | Yes                                        | Visual editor used for <br/>the input field.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
 | `pattern`    | String                                                                                                                                               | No                                         | Regular expression that will be <br/>used to validate the input. <br/> If validation fails, <br/>the Actor will not run.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
 | `minLength`  | Integer                                                                                                                                              | No                                         | Minimum length of the string.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
 | `maxLength`  | Integer                                                                                                                                              | No                                         | Maximum length of the string.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |