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

xtask/templates/admin/index.md

@@ -0,0 +1,7 @@
+# Admin Commands
+
+These are all the admin commands. TODO fill me out
+
+{%~ for category in categories %}
+- [`!admin {{ category.name }}`]({{ category.name }}/) {{ category.description }}
+{%- endfor %}