Contributing instructions: allow maintainers to push to forks, testsuite link

Description

• hopefully this simplifies the forking workflow
• when contributors allow it, maintainers can directly write to the source branch in a fork this way we can fix things more quickly and help with rebasing
• no additional MR in the openCFS repo is necessary

Check before assigning to a maintainer for review

• The history is clean
• New code is well documented and understandable
• There is a test case (in case of new functionality)
• Every commit passes the pipeline

Maintainer checks before merge

• Review is approved, and all comments are resolved
• Check for Testsuite changes, if yes check
  • Testsuite merge request merges fast-forward
  • Testsuite-submodule SHA of every CFS commit references a commit in Testsuite Merge Request
  • Testsuite-submodule SHA of last CFS commit points to HEAD of corresponding Testsuite branch
• Pipeline passes for every commit
  • all tests of the stage test pass
  • tests in the stage test_extra run
  • new tests are actually running (e.g. check if they appear on CDash)