Improve the management of contributions done from fork repositories

[tbouffard]

DISCLAIMER: this is a top level issue to track a global initiative

Rationale

More and more contributions done from fork repositories, even from people working at Bonitasoft.
At present, several CI tasks that are supposed to facilitate contribution management do not work for these contributions.
This confuses contributors rather than helping them, generating extra work for Bonitasoft engineers who have to explain the current behavior.

These issues must be handled to improve the situation. The most urgent tasks are about subjects generating false positives if any.

Sub-tasks

🚧 WIP: the list of subjects to handle is a work in progress 🚧

• Site preview build and xref validation not correctly done when the PR is created from a fork repository #402
• CI: From github action, add label when PR are opened #456
• The "Contribution Checks" fails to create PR comment when the PR is created from a fork repository #685
• Provide a live preview for contributions done from fork repo  #686