Make gitlab pages for governance.drupal.org

I've refactored the code to support GitLab Pages documentation based on the docs.

I'm not sure why pipelines isn't yet running on the MR. To test so far:

brew install material-mkdocs
mkdocs build

I made the pipeline run on the MR, not sure if I've seen this as a maintainer of the project only, but this is where it allowed me to run it:

Automatically closed - issue fixed for 2 weeks with no activity.