[CI] Include/exclude certain jobs for docs-only branches

Summary

If the branch name starts with docs- or ends with -docs, then certain jobs will be included or excluded.

Documentation-related tests will be added as part of #363 (closed).

Testing

Example docs branch pipeline: https://gitlab.com/gitlab-org/cloud-native/gitlab-operator/-/pipelines/382344977

Note: I wasn't able to add the docs branch condition to include[].template.rules, seemingly due to gitlab-org/gitlab#337633 (closed).

Closes #262 (closed)