feat: provide metamodel.yaml as Bazel target

Author: a-zw

docs/reference/bazel_macros.rst

@@ -81,6 +81,8 @@ Minimal example (root ``BUILD``)
 
   The custom ``metamodel.yaml`` must follow the same schema as the default one
   (see :doc:`score_metamodel </internals/extensions/metamodel>`).
+  You may use ``@score_docs_as_code//src/extensions/score_metamodel:metamodel_yaml``
+  for extension processing.
   When ``metamodel`` is omitted the default metamodel is used unchanged.
 
 Edge cases

src/extensions/score_metamodel/BUILD

@@ -34,6 +34,11 @@ filegroup(
     ),
 )
 
+filegroup(
+    name = "metamodel_yaml",
+    srcs = ["metamodel.yaml"],
+)
+
 filegroup(
     name = "all_sources",
     srcs = [