ci: Add Margaret as a reviewer automatically to any docs changes

[mattrunyon]

This will request @margaretkennedy as a reviewer any time a docs markdown file is changed without making her a codeowner and a required review.

It will not re-request review from her, so it will allow PR authors to make multiple commits and then re-request once they're ready

Example of the action working: mattrunyon-test-org/test-public#37

[mofojed]

Wonder if we could/should leverage using Teams in GitHub for assigning, instead of just assigning Margaret: https://docs.github.com/en/organizations/organizing-members-into-teams/managing-code-review-settings-for-your-team

[mattrunyon]

We could. I just did this as the easiest first iteration since it was only requested that Margaret be added.

I think the only modification would be checking teams instead of users in requested reviews and probably some other check in the submitted reviews.

I added logic to not re-request review if there's already a requested review or existing review. Once reviewed, Github does not list under requested reviews, so requestReviewers is equivalent to clicking the "re-request review" button. I didn't want that to be automatic so PR authors can re-request when they're ready