Add rbxtemp to valid url protocols (#878)

## Changes

Add documentation for the `rbxtemp` URL protocol that's used for
temporary

Fixes: [Content Data Types doesn't mention rbxtemp - Documentation
Issues](https://devforum.roblox.com/t/content-data-types-doesnt-mention-rbxtemp/457256)

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---------

Author: ccuser44

content/en-us/projects/assets/index.md

@@ -262,6 +262,12 @@ end
 
 <span><Chip label='rbxhttp://Thumbs/Avatar.ashx?x=100&y=100&format=png' color='primary' size='large' /></span>
 
+### rbxtemp
+
+`rbxtemp` allows you to refer `Class.File|Files`. Use `Class.File:GetTemporaryId()` to generate a valid `rbxtemp` URL. For example:
+
+<span><Chip label='rbxtemp://1' color='primary' size='large' /></span>
+
 ### https / http
 
 `https` or `http` points to the exact location of something on the internet. It only works on Roblox-approved domains and raises an error if you use it elsewhere.

content/en-us/reference/engine/datatypes/Content.yaml

@@ -39,12 +39,18 @@ description: |
               <td>Allows for easy loading of thumbnails.</td>
               <td>rbxthumb://type=Asset&id=24813339&w=150&h=150</td>
           </tr>
-      <tr>
+          <tr>
               <td><b>rbxhttp</b></td>
               <td>A path on the Roblox website</td>
               <td>Fetches content from the Roblox website.</td>
               <td>rbxhttp://Thumbs/Avatar.ashx?x=100&y=100&format=png</td>
           </tr>
+          <tr>
+              <td><b>rbxtemp</b></td>
+              <td>A temporary asset</td>
+              <td>Loads a temporary asset. Often generated by `Class.File:GetTemporaryId()`.</td>
+              <td>rbxtemp://1</td>
+          </tr>
           <tr>
               <td><b>https</b>/<b>http</b></td>
               <td>A website URL</td>