[red-knot] Move `UnionBuilder` tests to Markdown #38555

Job	Run time
Determine changes	8s
cargo fmt	18s
mkdocs	2m 31s
cargo build (release)	0s
pre-commit	3m 11s
python package	1m 20s
cargo build (msrv)	4m 19s
cargo fuzz build	5m 17s
cargo clippy	3m 42s
benchmarks	5m 2s
cargo shear	22s
cargo test (linux)	2m 34s
cargo test (linux, release)	3m 47s
cargo test (wasm)	1m 50s
cargo test (windows)	5m 22s
formatter instabilities and black similarity	0s
test scripts	1m 2s
test ruff-lsp	33s
ecosystem	2m 40s
fuzz parser	0s
	43m 58s

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