Add Gitlab Contribution Cost Factor Docs and Remove flag

Now ready to merge as verification term is complete. Flags Issues:

• #365862 (closed) (feature flag)

• #367692 (closed) (long lived :ops flag)

What does this MR do and why?

Add documentation and remove the flag for the gitlab community contribution cost factor.

Related MR: !89742 (merged)

Related Issue: #365447 (closed)

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