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/_nav.json

@@ -18,7 +18,7 @@
             },
             {
                 "text": "Admin Command Reference",
-                "link": "/reference/admin"
+                "link": "/reference/admin/"
             },
             {
                 "text": "Server Reference",