add small script + github action to check that public docs updated

[joeyorlando]

What this PR does

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.

No changes made to "engine" or "grafana-plugin" directories

Job skipped when "no public docs" label applied to PR

Changes made to "engine" and/or "grafana-plugin" directories and public docs also updated

Changes made to "engine" and/or "grafana-plugin" directories but public docs were not updated

Which issue(s) this PR fixes

N/A

Checklist

• Tests updated (N/A)
• Documentation added (N/A)
• CHANGELOG.md updated (N/A)