Exclude db/types.py from sphinx-apidoc target

SQLAlchemy 1.4.27 released on Nov 11, and tox-docs job error become
to occur since then.
`:ref:` attribute introduced from 1.4.27 in docstring of the parent
class of our TypeDecorator cannot be referred.
This patch fixes the error by excluding db/types.py from target of
sphinx-apidoc.

Change-Id: If7944774f2336d8f98124e9934da47b11801c607

doc/source/conf.py

@@ -49,6 +49,9 @@ apidoc_excluded_paths = [
     'vnfm/workflows',
     'db/migration/alembic_migrations/*',
     'db/migration/alembic_migrations',
+    # TODO(ueha): Excluding 'db/types.py' will be removed after the issue of
+    # TypeDecorator is fixed.
+    'db/types.py',
     'hacking']
 apidoc_separate_modules = True