docs: mention `git pack-refs` for co-located repos #6890

Job	Run time
build (ubuntu-latest)	8m 12s
Check protos	52s
build (macos-13)	6m 11s
Check that MkDocs can build the docs	23s
build (windows-latest)	8m 27s
build (ubuntu-latest, --all-features)	7m 6s
Check formatting	18s
cargo-deny (advisories)	29s
Clippy check	3m 22s
cargo-deny (bans licenses sources)	31s
	35m 51s

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