[FEATURE] Documentation: add git guidelines for code-authorities.

[thibsy]

Hi @ILIAS-eLearning/technical-board,

I would like to propose some git guidelines. Since there are a lot of new authorities, who can sign off on code changes, I think now is a good time to do so. The guidelines I wrote down primarily focus on:

• how we write commit messages and make them meaningful
• how we sign off on code changes from pull requests
• how we integrate changes into different branches of the repository

All of which hopefully improve consistency and transparency. These guidelines are already seasoned and battle-tested. We use most of them inside the UI framework for some time now, and at sr solutions we live by the commit messages for even longer. I had to update some aspects to fit ILIAS more generally though, something very new-ish is the part about signatures when approving changes.

I am looking forward to your feedback.

Kind regards,
@thibsy