Add stage for base-files pipeline - T8977

The base-files package is special, as in it will automatically create the right MR for the respective release.

To do so, the pipeline needs the RELEASE-package-source-builder docker image to have been built and ready.

This proposed MR introduces the stage in the Release Branching pipeline, documenting the same.

Signed-off-by: Ritesh Raj Sarraf ritesh.sarraf@collabora.com