Update documentation to reflect the discontinuation. #5790

Job	Run time
druid doctests	1m 8s
mdbook build	1m 42s
cargo fmt	24s
Docs (windows-latest)	2m 4s
Docs (macos-latest)	1m 8s
cargo clippy+test (windows-latest)	3m 17s
cargo clippy+test (gtk)	3m 42s
Docs (ubuntu-latest)	3m 46s
cargo clippy+test (macos-latest)	2m 16s
cargo test (wasm32)	3m 1s
cargo clippy+test (ubuntu-latest)	3m 21s
cargo clippy+test (wayland)	2m 48s
	28m 37s

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