Clarify Contributing guidelines

Manuel Grabowski – PTO until 2026-01-01requested to merge
mg-contrib-guideline-20250430 into main
  • A leftover link to the old experimental path
  • A few rewordings
  • Trying to get rid of any "success"/"fail" mentions for checks as we aim to settle on using just "passed" / "not passed"
  • Add specific /etc/gitlab/gitlab*rb guidance
  • Mention a few tips gathered while reviewing !75 (merged)
    • Single/double quotes
    • Show a common RegEx for commented lines
    • Mention -sq for grep

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