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/_meta.json

@@ -8,10 +8,5 @@
         "type": "file",
         "name": "admin",
         "label": "Admin Commands"
-    },
-    {
-        "type": "file",
-        "name": "server",
-        "label": "Server command"
     }
 ]