Add GitHub Actions for PR comments and migration documentation

[KevinMind]

Fixes: mozilla/addons#15559

Description

Add documentation for migrations and a workflow to comment a link to those docs on opened/reopened PRs that modify migration files.

Context

Every time github syncs a PR based on master, it will check if files changed in **/migrations/*.py. If it finds any of those files, it will add a comment on the PR linking to our migration docs and indicating caution should be taken to write safe and stable migrations.

Testing

Modifed a migration file and triggered the workflow, you can see the comment on the PR

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.

[github-actions]

⚠️ Data migration detected ⚠️

It looks like there are data migrations in your PR.
Please read our policies on database migrations before merging.

Migration Docs