docs: Add coverage for named directory icon support (#39387)

Also updates the link to the new schema version which now includes named
directory icons.

Release Notes:

- N/A

docs/src/extensions/icon-themes.md

@@ -13,13 +13,13 @@ There are two important directories for an icon theme extension:
 - `icon_themes`: This directory will contain one or more JSON files containing the icon theme definitions.
 - `icons`: This directory contains the icons assets that will be distributed with the extension. You can created subdirectories in this directory, if so desired.
 
-Each icon theme file should adhere to the JSON schema specified at [`https://zed.dev/schema/icon_themes/v0.2.0.json`](https://zed.dev/schema/icon_themes/v0.2.0.json).
+Each icon theme file should adhere to the JSON schema specified at [`https://zed.dev/schema/icon_themes/v0.3.0.json`](https://zed.dev/schema/icon_themes/v0.3.0.json).
 
 Here is an example of the structure of an icon theme:
 
 ```json
 {
-  "$schema": "https://zed.dev/schema/icon_themes/v0.2.0.json",
+  "$schema": "https://zed.dev/schema/icon_themes/v0.3.0.json",
   "name": "My Icon Theme",
   "author": "Your Name",
   "themes": [
@@ -30,6 +30,12 @@ Here is an example of the structure of an icon theme:
         "collapsed": "./icons/folder.svg",
         "expanded": "./icons/folder-open.svg"
       },
+      "named_directory_icons": {
+        "stylesheets": {
+          "collapsed": "./icons/folder-stylesheets.svg",
+          "expanded": "./icons/folder-stylesheets-open.svg"
+        },
+      }
       "chevron_icons": {
         "collapsed": "./icons/chevron-right.svg",
         "expanded": "./icons/chevron-down.svg"