Documentation: Please add pages for basic tasks like "How to add a repo", "How to add a new branch protection / repository rule"

[fredg02]

It would be nice to be able to point projects to such examples when they ask about it in HelpDesk issues.

E.g. https://gitlab.eclipse.org/eclipsefdn/helpdesk/-/issues/5471#note_2969340

[netomi]

first additions in 0beded8e1b7052de3d9ec4428be0af3b775ac5ef

[netomi]

There is now:

• https://otterdog.readthedocs.io/en/latest/userguide/adding/
• https://otterdog.readthedocs.io/en/latest/userguide/removal/