triage: improvements to issues workflow
Define a workflow for the issues to try getting more help on triage and investigation.
Add explanation of how to indicate the new issues workflow to MAINTAINERS.md: triage -> investigation -> devel. The different stages are indicated using labels, as Gitlab doesn't have anything better for it (it has scoped labels but they are not available in the free version).
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-wanted::{triage, investigation, devel} and unassigned. It will also ensure that mutually exclusive labels are not used at the same time.
The labels help-wanted::* 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.
Note: there are some additional things that need to be done if this is approved/merged:
- Create the labels in the Gitlab's project
- Configure the scheduled pipelines in Gitlab
- Test the issues automation after that, and remove the
--dry-runif it's OK.