Do not release if the docs are not up-to-date

[ctreatma]

This repo and others created from it rely on PRs to generate updated docs. After every PR is merged, the docs update job runs; that job regenerates the docs and opens a PR if any changes are needed.

The PR workflow for docs means that it's possible to release code changes before the corresponding docs changes have been merged. To protect against this, we could update the release workflow to check if the docs are outdated, and fail with an informative error if the docs need changes.