CI job to push automated fixes to PRs

We could have a CI workflow that could fix some semi-automated chores you sometimes need to do for PRs. Ideally it would only run when explicitly asked, e.g. when a PR label is set? It could do the following things:

• Update link references in CHANGELOG.md
• Rebuild the theme CSS files.
• Push code formatter fixes (we only have prettier right now, but at some point we might also have JuliaFormatter)

As a proof of concept, this prettier actions does that for prettier formatting checks: https://github.com/creyD/prettier_action

This would be a good first issue for anyone who wants to learn how to write slightly non-trivial GitHub Actions workflows.