meta: add initial `GOVERNANCE.md` #16918

Job	Run time
build (ubuntu-latest)	5m 7s
build (macos-13)	10m 4s
Build jj-lib without Git support	46s
build (macos-14)	3m 55s
build (windows-latest)	11m 42s
build (ubuntu-latest, --all-features)	6m 0s
cargo-deny (advisories)	24s
cargo-deny (bans licenses sources)	25s
Check protos	22s
Clippy check	2m 37s
Check that MkDocs can build the docs	10s
Check that MkDocs can build the docs with latest Python and uv	11s
Check formatting	15s
	41m 58s

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