Add docs page about desktop icons

docs/docs/usage/desktop-icons.md

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+
+# Desktop Icons
+
+Support for displaying desktop entries (icons) is built-in.
+
+## How do I control whether desktop icons appear?
+
+With the React configuration, desktop icons are displayed if you render the
+`<DesktopEntries />` component.
+
+## How do I specify which icons appear?
+
+The window manager looks for `.desktop` files in your XDG Desktop user
+directory. This is usually `$HOME/Desktop` (`~/Desktop`) but is configurable [as
+described here](https://wiki.archlinux.org/title/XDG_user_directories).
+
+Normally, installed packages create `.desktop` files in a system folder; they
+aren't usually put into your XDG Desktop directory. This means that you should
+copy the `.desktop` files that you actually want to appear into your XDG Desktop
+directory to get them to appear in the window manager.
+
+The Arch Linux wiki [mentions](https://wiki.archlinux.org/title/desktop_entries#Application_entry)
+some common locations for `.desktop` files, but it may vary.
+
+## What goes in a `.desktop` file?
+
+The intention is to support [standard desktop files](https://wiki.archlinux.org/title/desktop_entries),
+but here are some notes about what is supported currently:
+
+### Supported properties
+
+- `NoDisplay` and `Hidden` suppress the display of a `.desktop` file.
+- `Name`: The desktop icon caption
+- `Type`: "Application" or "Link"
+- `Icon`: SVG or PNG icon
+
+For type Application:
+
+- `Exec`: Application to launch
+- `Path`: Working directory when launching the application
+
+For type Link:
+
+- `URL`: URL to open