Document labels that are used for triaging GitHub issues in the bicep repo

As part of @asilverman's work to improve our auto-detection of stale issues, we ran into some inconsistencies in our use of labels. To help clarify, we should document all the labels that are used, what they mean, and delete any labels that we haven't ever or no longer use (i.e. `Needs: Author Feedback`

No documentation file or test is named. Start by reviewing the Bicep repository's current GitHub labels and how they are used for issue triage; document each label's meaning and identify labels that have never or no longer been used, including `Needs: Author Feedback`. Done means the label guidance is recorded and obsolete labels are removed.

Written by the indexing model from the issue text.