docs: Add documentation for icon theme (#25973)

Adds documentation for the icon theme setting (mostly based on the
documentation from theme but adjusted for icon theme).

Release Notes:

- N/A

---------

Co-authored-by: Peter Tripp <peter@zed.dev>
Co-authored-by: Marshall Bowers <git@maxdeviant.com>

docs/src/configuring-zed.md

@@ -1539,6 +1539,78 @@ To interpret all `.c` files as C++, files called `MyLockFile` as TOML and files
 
 `boolean` values
 
+## Icon Theme
+
+- Description: The icon theme setting can be specified in two forms - either as the name of an icon theme or as an object containing the `mode`, `dark`, and `light` icon themes for files/folders inside Zed.
+- Setting: `icon_theme`
+- Default: `Zed (Default)`
+
+### Icon Theme Object
+
+- Description: Specify the icon theme using an object that includes the `mode`, `dark`, and `light`.
+- Setting: `icon_theme`
+- Default:
+
+```json
+"icon_theme": {
+  "mode": "system",
+  "dark": "Zed (Default)",
+  "light": "Zed (Default)"
+},
+```
+
+### Mode
+
+- Description: Specify the icon theme mode.
+- Setting: `mode`
+- Default: `system`
+
+**Options**
+
+1. Set the icon theme to dark mode
+
+```json
+{
+  "mode": "dark"
+}
+```
+
+2. Set the icon theme to light mode
+
+```json
+{
+  "mode": "light"
+}
+```
+
+3. Set the icon theme to system mode
+
+```json
+{
+  "mode": "system"
+}
+```
+
+### Dark
+
+- Description: The name of the dark icon theme.
+- Setting: `dark`
+- Default: `Zed (Default)`
+
+**Options**
+
+Run the `icon theme selector: toggle` action in the command palette to see a current list of valid icon themes names.
+
+### Light
+
+- Description: The name of the light icon theme.
+- Setting: `light`
+- Default: `Zed (Default)`
+
+**Options**
+
+Run the `icon theme selector: toggle` action in the command palette to see a current list of valid icon themes names.
+
 ## Inlay hints
 
 - Description: Configuration for displaying extra text with hints in the editor.