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/token.md

@@ -0,0 +1,16 @@
+# `!admin token`
+
+- Commands for managing registration tokens
+
+
+## `!admin token issue`
+
+- Issue a new registration token
+
+## `!admin token revoke`
+
+- Revoke a registration token
+
+## `!admin token list`
+
+- List all registration tokens