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Add Copilot instructions for FFs, and update documentation more generally #13690

Job	Run time
check-codeql-versions	25s
analyze-other (actions)	47s
analyze-javascript (ubuntu-22.04, linked)	3m 4s
analyze-javascript (macos-14-xlarge, linked)	2m 54s
analyze-javascript (macos-15-xlarge)	3m 5s
analyze-javascript (windows-2025, linked)	5m 57s
analyze-javascript (ubuntu-24.04, linked)	3m 42s
analyze-javascript (ubuntu-22.04)	4m 34s
analyze-javascript (macos-14-xlarge)	2m 33s
analyze-javascript (ubuntu-24.04)	3m 36s
analyze-javascript (macos-15-xlarge, linked)	3m 7s
analyze-javascript (windows-2022, linked)	5m 29s
analyze-javascript (windows-2022)	5m 24s
analyze-javascript (windows-2025)	6m 59s
	51m 36s