build

docs: mention that that Rust 1.76 is required to install from source #15209

Job	Run time
Build jj-lib without Git support	43s
Check protos	30s
cargo-deny (advisories)	23s
build (ubuntu-latest)	4m 41s
Check that MkDocs can build the docs	21s
cargo-deny (bans licenses sources)	22s
Clippy check	2m 35s
build (macos-13)	5m 17s
build (macos-14)	4m 43s
Check that MkDocs can build the docs with Poetry 1.8	25s
build (windows-latest)	10m 53s
Check formatting	19s
build (ubuntu-latest, --all-features)	5m 38s
	36m 50s

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