Add a tarball-based documentation-building job

Add a tarball-based documentation-building job

The existing "docs" GitLab CI job operates on a Git repository rather
than a source tarball.  This prevents it from detecting issues caused by
files missing from source tarballs.  Add a new GitLab CI job similar to
the "docs" one, but using a source tarball rather than a Git repository.
Extract YAML bits used by multiple job definitions into anchors to avoid
code duplication.  Drop the "allow_failure: false" key in the process as
it is the implicit default for non-manual jobs.  Replace the
"artifacts:paths" key with "artifacts:untracked" in order to include all
untracked files in the artifact archive for each documentation-building
job; this allows tarball-based artifacts to be properly captured and
also facilitates troubleshooting failed jobs.