Rename deploy.yml to gh-pages.yml - that's what it does (#2857)

## Summary:

This PR moves the `deploy.yml` workflow file to `gh-pages.yml`. Over in Khan/perseus we [did the same](Khan/perseus#2993) mostly because `deploy` sounds like we're doing a release and it wasn't clear what exactly the deploy was doing. Happy to not land this but I think cross-repo consistency is good (and we do have quite a bit between WB and Perseus, which is useful IMHO).

Issue: LEMS-3681

## Test plan:

I can't see how renaming a workflow file will break anything. I'll land this and then watch the Actions tab for a bit...

Author: jeremywiebe

Reviewers: beaesguerra, jandrade, marcysutton

Required Reviewers:

Approved By: beaesguerra

Checks: ✅ 11 checks were successful, ⏹️  2 checks were cancelled, ⏭️  5 checks have been skipped

Pull Request URL: #2857

Author: jeremywiebe