markdown cleanup (bookmarks.md, contributing.md) #16442

Job	Run time
Check protos	25s
Build jj-lib without Git support	45s
build (ubuntu-latest)	5m 10s
Check that MkDocs can build the docs	12s
build (macos-13)	12m 23s
Check that MkDocs can build the docs with latest Python and uv	10s
build (macos-14)	3m 43s
Check formatting	19s
cargo-deny (advisories)	24s
build (windows-latest)	11m 0s
Clippy check	2m 40s
cargo-deny (bans licenses sources)	23s
build (ubuntu-latest, --all-features)	6m 9s
	43m 43s

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