docs/CONTRIBUTING: explain various labels and add link to "reviewable" PRs

docs/CONTRIBUTING: explain various labels and add link to "reviewable" PRs

The linked filter gives an up-to-date list of pull requests that need review.
(Yes, there's too many.) We used to set 'needs-review' label, but that is
not available to non-members, and also every pull requests which is not labeled
'reviewed/needs-rework'/'ci-fails/needs-rework'/'needs-rebase' can and should
be reviewed.

If this is merged, I'll drop the 'needs-review' label.