faq: add an entry about why merge commits are often empty #16619

Job	Run time
Build jj-lib without Git support	50s
cargo-deny (advisories)	22s
build (ubuntu-latest)	5m 9s
Check protos	31s
cargo-deny (bans licenses sources)	25s
build (macos-13)	10m 16s
Clippy check	2m 33s
build (macos-14)	4m 4s
build (windows-latest)	10m 29s
Check that MkDocs can build the docs with latest Python and uv	20s
build (ubuntu-latest, --all-features)	6m 7s
Check formatting	18s
Check that MkDocs can build the docs	16s
	41m 40s

Provide feedback