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/appservices.md

@@ -0,0 +1,28 @@
+# `!admin appservices`
+
+- Commands for managing appservices
+
+
+## `!admin appservices register`
+
+- Register an appservice using its registration YAML
+
+This command needs a YAML generated by an appservice (such as a bridge), which must be provided in a Markdown code block below the command.
+
+Registering a new bridge using the ID of an existing bridge will replace the old one.
+
+## `!admin appservices unregister`
+
+- Unregister an appservice using its ID
+
+You can find the ID using the `list-appservices` command.
+
+## `!admin appservices show-appservice-config`
+
+- Show an appservice's config using its ID
+
+You can find the ID using the `list-appservices` command.
+
+## `!admin appservices list-registered`
+
+- List all the currently registered appservices