This PR adds a new GitHub Action which will run on PRs against `dev` and
`main`. The GitHub action will not run if the label of "no public docs"
has been applied to the PR in question:
Otherwise, it will check to see if any changes were made to either the
`engine` or `grafana-plugin` directories. If so, it will then check
whether changes were also made to the `docs` directory. If not, it will
fail the job and block the build.
