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/src/tasks/mod.rs

@@ -0,0 +1,37 @@
+type TaskResult<T> = Result<T, Box<dyn std::error::Error>>;
+
+#[macro_export]
+macro_rules! tasks {
+    (
+        $(
+            $module:ident: $desc:literal
+        ),*
+    ) => {
+        $(pub(super) mod $module;)*
+
+        #[derive(clap::Subcommand)]
+        #[allow(non_camel_case_types)]
+        pub(super) enum Task {
+            $(
+                #[clap(about = $desc, long_about = None)]
+                $module($module::Args),
+            )*
+        }
+
+        impl Task {
+            pub(super) fn invoke(self, common_args: $crate::Args) -> TaskResult<impl std::process::Termination> {
+                match self {
+                    $(
+                        Self::$module(task_args) => {
+                            $module::run(common_args, task_args)
+                        },
+                    )*
+                }
+            }
+        }
+    };
+}
+
+tasks! {
+	generate_docs: "Generate various documentation files. This is run automatically when compiling the website."
+}