Add explanation of how to indicate the new issues workflow to
MAINTAINERS.md: triage -> investigation -> devel. The different
stages are indicated using Gitlab's scoped labels (mutually exclusive).

These stages try to hightlight that the issue cannot be fixed and it's
not moving forward because more info is needed, already.  Also, add a
section to CONTRIBUTING.md highlighting the importance of helping in
the triage and investigation stages: developers often cannot fix bugs
because lack of time to investigate, but even users that doesn't know
how to fix it due to lack of knowledge of the code base can help thanks
to their knowledge on networking.

Finally, make the 'triage:issues' CI job to work again, adding some
new policies with new automations. The automation will add or remove the
labels: stale, help-needed::{triage, investigation, devel} and
unassigned.

The labels help-needed::* and unassigned will be automatically added to
all issues without an assignee. This reflects better the reality of not
having enough time to work on most of the issues unless there is some
external help.