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/check.md

@@ -0,0 +1,8 @@
+# `!admin check`
+
+- Commands for checking integrity
+
+
+## `!admin check check-all-users`
+
+_(no description)_