Clarify issue/PR choreography

From a discussion with @picnixz:

I think the problem with the devguide is the first sentence is "Create an issue that describes the change" and you'll see the mention to DPO only during creation. We can improve that
I would expect people to read the issue template correctly and understand that something is not trivial
and more importantly: WAIT for triagers to decide whether it's ok or not as an issue
There are obvious cases where the issue doesn't a triager for validation, like a clear bug (and even this is subject to interpretation sometimes) but the process issue+PR is the worst for triagers, especially when the PR is automated.

This might be a duplicate of other feedback here in issues, but I wanted to capture it before going back to $WORK.

Commencez par localiser les instructions d’ouverture du devguide et le modèle d’issue, puis examinez où sont introduites les consignes concernant DPO et les triagers. Précisez que les contributors doivent attendre la validation d’un triager avant d’ouvrir une PR, tout en signalant la discussion autour des bugs évidents. C’est terminé lorsque le workflow issue-to-PR et les exceptions sont explicites pour les contributors.