Document the script mode (zauberzeug#5299)

### Motivation

We can't talk about not using the script mode in
zauberzeug#5297, if we did not document
what that even is.

### Implementation

Add a small section on what is script mode in the documentation. 

### Progress

- [x] I chose a meaningful title that completes the sentence: "If
applied, this PR will..."
- [x] The implementation is complete.
- [x] Pytests have been added (or are not necessary).
- [x] Documentation has been added (or is not necessary).

---------

Co-authored-by: Falko Schindler <[email protected]>

Author: evnchn

website/documentation/content/section_pages_routing.py

@@ -21,6 +21,19 @@
 doc.intro(sub_pages_documentation)
 
 
+@doc.demo('Script Mode', '''
+    While generally you would either use `@ui.page` decorators or a root function to create pages,
+    it is cumbersome when making quick prototypes or demos.
+    In such cases, you can use "script mode" by simply writing code at the top level of a script.
+    The code will be executed once per client connection, and the interface will be created for that client.
+''')
+def script_mode_demo():
+    ui.label('No @ui.page, no root function, but still a working page!')
+
+
+doc.text('', 'Note: Many of the demos in this documentation are written in script mode for conciseness.')
+
+
 @doc.auto_execute
 @doc.demo('Parameter injection', '''
     Thanks to FastAPI, a page function accepts optional parameters to provide