Document the `triaged` label

[ezio-melotti]

The triaged label was recently added, and should be documented.

See:

• Remove the "invalid" label, add a "valid" label core-workflow#465
• Adding a new "validation" step to our issue handling workflow steering-council#130
• https://discuss.python.org/t/improve-communication-with-contributors-on-the-issue-tracker-and-prs-language-summit-follow-up/15812/30

I can take care of this while updating the other labels (see #821).