Add a GitHub workflow that can automatically update TOCs in README files or fail if not updated locally.

[davereid]

We should have an automated way to catch if the README.md file is updated with new headings, but the automatic DocToc table of contents hasn't been changed.

Or alternatively, we could have a workflow that automatically adds a commit to the PR that fixes it automatically. https://github.com/marketplace/actions/toc-generator could potentially be used for this, but we would need to provide a GITHUB_TOKEN that is generated using our GitHub app so that workflows will run after the commit.