Split mkdocs build and deploy into separate jobs

@alexdmccabe there's now a merge conflict that I think you'll be able to resolve more efficiently than I.

Ideally, the changes would be deployed using GitLab Pages to a separate URL that doesn't overwrite the deployment from the default branch, but I don't know if this is possible.

I think we saw this work recently here 📌 Set up MkDocs for docs.memberplatform.org Active , no?

Conflict fixed. Ready for re-review.

Very nifty, thanks!

I tried to find a way to get the preview URL bubbled up somewhere more discoverable than the job logs, but failed :(

I might have figured that out with the environment key in the GitLab CI/CD YAML.

I'll open a follow-up issue.