feat: Improve admin command reference generation

- Change xtasks to use `clap` for argument parsing
- Generate admin command reference manually instead of with `clap_markdown`
- Split admin command reference into multiple files

docs/reference/admin/index.md

@@ -0,0 +1,14 @@
+# Admin Commands
+
+These are all the admin commands. TODO fill me out
+
+- [`!admin appservices`](appservices/) - Commands for managing appservices
+- [`!admin users`](users/) - Commands for managing local users
+- [`!admin token`](token/) - Commands for managing registration tokens
+- [`!admin rooms`](rooms/) - Commands for managing rooms
+- [`!admin federation`](federation/) - Commands for managing federation
+- [`!admin server`](server/) - Commands for managing the server
+- [`!admin media`](media/) - Commands for managing media
+- [`!admin check`](check/) - Commands for checking integrity
+- [`!admin debug`](debug/) - Commands for debugging things
+- [`!admin query`](query/) - Low-level queries for database getters and iterators