build

Add a paragraph on how to install jj completions into Powershell. #17432

Job	Run time
Check protos	32s
Build jj-lib without Git support	44s
Check formatting	16s
build (ubuntu-latest)	5m 17s
build (macos-13)	9m 23s
build (macos-14)	4m 33s
build (windows-latest)	9m 51s
build (ubuntu-latest, --all-features)	6m 1s
cargo-deny (advisories)	25s
cargo-deny (bans licenses sources)	22s
Run doctests	2m 21s
Clippy check	2m 51s
Check that MkDocs can build the docs	13s
Check that MkDocs can build the docs with latest Python and uv	12s
	43m 1s

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