docs/lib: generalise menu impl using module system

Move the mdbook menu rendering code into the module system and
generalise it to apply to multiple "categories" (mdbook parts) and
"types" of category (prefix, suffix, etc).

docs/modules/menu.nix

@@ -0,0 +1,43 @@
+{
+  lib,
+  config,
+  options,
+  ...
+}:
+let
+  categoryType = lib.types.submoduleWith {
+    modules = [ ./category.nix ];
+  };
+
+  categories = builtins.removeAttrs config (builtins.attrNames options);
+in
+{
+  freeformType = lib.types.attrsOf categoryType;
+
+  options._menu = {
+    text = lib.mkOption {
+      type = lib.types.str;
+      description = "The rendered menu.";
+      readOnly = true;
+    };
+  };
+
+  config._menu = {
+    text = lib.pipe categories [
+      builtins.attrValues
+      (map (x: x._category))
+      (lib.sortOn (x: x.order))
+      (builtins.groupBy (x: x.type))
+      (
+        {
+          prefix ? [ ],
+          normal ? [ ],
+          suffix ? [ ],
+        }:
+        prefix ++ normal ++ suffix
+      )
+      (map (x: x.text))
+      (builtins.concatStringsSep "\n\n")
+    ];
+  };
+}