Preventing maintainer docs from automerging if marked with "do-not-merge" label

[WhitWaldo]

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Description

Adding guard to prevent PRs from merging that would otherwise be acceptable when they're marked with the "do-not-merge" label. Automatically runs whenever a label is removed in case the do-not-merge is removed (prompting immediate merge if conditions are acceptable).

Further, if a non-maintainer opens a PR against docs that's restricted only to a single SDK directory, a maintainer of that group can add their "automerge: " label to it and that'll suffice to trigger the bot and allow automerge. There are some variations made on this theme:

• If an "automerge: " label is applied and the changes in the PR don't correspond to the directory of the applied label, the label will be removed and the automerge will not happen.
• If an SDK maintainer comes along and applies an "automerge: " label for an SDK they're not a maintainer of, the label will be removed and the automerge will not happen
• If anyone at all comes along and applies an "automerge: " label and they're not part of the maintainer group corresponding with the label, the label will be removed and the automerge will not happen.

Issue reference