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Add Copilot instructions for FFs, and update documentation more generally #402

Job	Run time
Per-language bundles (java, ubuntu-latest, nightly-latest, none)	55s
Per-language bundles (swift, macos-latest-xlarge, nightly-latest, autobuild)	7m 28s
Per-language bundles (javascript, ubuntu-latest, nightly-latest)	1m 0s
Per-language bundles (go, ubuntu-latest, nightly-latest, autobuild)	1m 20s
Per-language bundles (python, ubuntu-latest, nightly-latest)	50s
Per-language bundles (ruby, ubuntu-latest, nightly-latest)	48s
Per-language bundles (cpp, ubuntu-latest, nightly-latest, manual, gcc -o main main.c)	53s
Per-language bundles (actions, ubuntu-latest, nightly-latest, actions javascript)	47s
Per-language bundles (csharp, ubuntu-latest, nightly-latest, none)	1m 4s
Per-language bundles (rust, ubuntu-latest, nightly-latest)	1m 49s
	16m 54s