build

cargo: enable vendored-libgit2, document how to properly dynamically link libgit2 #12804

Job	Run time
Build jj-lib without Git support	44s
Check protos	36s
build (ubuntu-latest)	4m 24s
build (macos-14)	4m 38s
Check formatting	17s
build (windows-latest)	10m 6s
build (ubuntu-latest, --all-features)	5m 23s
Clippy check	2m 34s
cargo-deny (advisories)	27s
cargo-deny (bans licenses sources)	21s
Check that MkDocs can build the docs	31s
Check that MkDocs can build the docs with Poetry 1.8	22s
	30m 23s

Provide feedback