meta: add initial `GOVERNANCE.md` #16917

Job	Run time
Check protos	24s
Check formatting	17s
Build jj-lib without Git support	50s
build (ubuntu-latest)	4m 58s
cargo-deny (advisories)	26s
Clippy check	2m 30s
build (macos-13)	5m 45s
cargo-deny (bans licenses sources)	23s
Check that MkDocs can build the docs	8s
build (macos-14)	3m 42s
Check that MkDocs can build the docs with latest Python and uv	11s
build (windows-latest)	10m 53s
build (ubuntu-latest, --all-features)	5m 56s
	36m 23s

Provide feedback