Define and add new process labels and document general label usage in CONTRIBUTING.md

[handrews]

Splitting out from #3509 so I can close that massive issue, in a recent(ish) TDC call, we agreed to the following (leaving out points that I already did the day of the call):

• For now, use review to bring issues to the TSC's attention for decision on whether to implement or not - in particular, if we aren't going to do something, we should close it
• @baywet noted that for things that are approved, a help wanted label is a common practice
• @karenetheridge mentioned some other options in additon to (or instead of?) the review, specifically triage and discussion-needed

@lornajane commented:

If we can nail down when to apply each of those, we can put something in the contributing guide and create those labels

This issue tracks that so it's not buried iin the mega-label-refactor issue, which can now be closed.