[Meta] Improve CI/CD

Would it make sense to parallelize the "test" stage next to the "build" stage (and have "deploy" stage dependent on the other two)? Won't make much difference for a while, but in theory? Also wonder whether "build" should be "build-docs" and "deploy" should be "deploy-docs" given we will eventually have other tasks that could probably be performed in parallel. I'm undoubtedly over-optimizing...

I updated the pipeline to use the Drupal template. If mkdocs.yml exists pages will be built and deployed, but only when the branch is the default. MR will not build or deploy pages.