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Add Copilot instructions for FFs, and update documentation more generally #9424

Job	Run time
Ruby analysis (macos-latest, linked)	1m 18s
Ruby analysis (ubuntu-latest, linked)	37s
Ruby analysis (macos-latest, nightly-latest)	1m 8s
Ruby analysis (ubuntu-latest, default)	42s
Ruby analysis (ubuntu-latest, nightly-latest)	37s
Ruby analysis (macos-latest, default)	1m 19s
	5m 41s