Clarify contribution rules

[duboism]

This issue discusses how we want to manage contribution:

• categories of contributors and their role
• PR approval (e.g. enforce reviews)

[duboism]

From my limited experience, there are 2 kind of contributors:

• Regular contributors/maintainers which are mostly GWOSC people.
• Occasional contributors: teachers, local organizing committee tutors or people interested into a specific topic (for instance, this year Continuous Waves). Those contributors may change from time to time, for instance each year so we have to manage that.

As always for GitHub, I would bend for assigning roles to teams, not individuals.

Also, in the traditional fork/PR model contributors fork the repository, work on their fork and then create a PR. This model is fine but it requires some git/GitHub skills that tutors or mentors may not possess (branches, juggling with several remotes, etc.). Also, if several people want to work on the same branch, the owner of the fork needs to invite them on their fork.

So I would create 2 teams:

• One which mainly reflects the GWOSC with "admin" rights (i.e. they can do everything including deleting the repo). Let's call this team core.
• One for contributors with a more limited role. They should however be able to write on the repository ("write" should be enough). Let's call this team contributors. We have to invite/remove people from this team accordingly.

Also, we can combine that with main branch protection and CODEOWNERS file.

The idea behind this is to simplify the contributions:

• People in the contributors team can create their own branch directly on the main repository (here) and work together without effort and open a PR. However, they can't approve the PR. Inviting people in the team is the occasion to onboard people when preparing an ODW. There is a slight risk of people modifying/destroying other branches (and then other people work).
• People in the core team can do the same and approve PR.

Note that this doesn't prevent the classic fork/PR contribution.

[duboism]

In order to test this, we contacted ODW 2025 mentors to add them in the contributors list.

Therefore, I added a branch protection rule for main (see https://github.com/gw-odw/odw/settings/branches). Maybe I'll add a CODEOWNERS file.