📝 Docs: Make 'status: triage' label & corresponding docs

[Josh Goldberg ✨]

Documentation Request Checklist

• I have read and agree to Mocha's Code of Conduct and Contributing Guidelines
• I have searched for related issues and issues with the faq label, but none matched my issue.
• I want to provide a PR to resolve this

Overview

We talked internally about how there's no single label for "this hasn't yet been (re-)triaged", and that's irksome for searching issues/PRs. There is an existing status: in review label but that's just for PRs.

We'll add a new label (or re-use status: needs review?) and update https://github.com/mochajs/mocha/blob/master/.github/CONTRIBUTING.md accordingly.

Additional Info

We'll want to be very clear that issues should only have a non-draft PR sent for either teeny docs fixes or issues accepting PRs.