Improve egui_kittest documentation and update CONTRIBUTING.md #13085

Job	Run time
Format + check + test	2m 33s
Check wasm32 + wasm-bindgen	2m 22s
cargo-deny aarch64-apple-darwin	42s
cargo-deny aarch64-linux-android	40s
android	27s
cargo-deny i686-pc-windows-gnu	42s
cargo-deny i686-pc-windows-msvc	39s
Check wasm32+atomics	1m 20s
ios	19s
cargo-deny i686-unknown-linux-gnu	42s
cargo-deny wasm32-unknown-unknown	41s
Run tests	1m 42s
cargo-deny x86_64-apple-darwin	40s
Check Windows	2m 10s
cargo-deny x86_64-pc-windows-gnu	40s
cargo-deny x86_64-pc-windows-msvc	41s
cargo-deny x86_64-unknown-linux-gnu	40s
cargo-deny x86_64-unknown-linux-musl	40s
cargo-deny x86_64-unknown-redox	41s
	19m 1s

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