Add an example teaching users about custom relationships #56275

Job	Run time
build (windows-latest)	3m 4s
check-doc	9m 25s
build (ubuntu-latest)	3m 19s
build (macos-latest)	3m 3s
check-missing-examples-in-docs	29s
ci	2m 30s
miri	10m 56s
run-examples-macos-metal	4m 46s
typos	7s
toml	1m 59s
check-bevy-internal-imports	5s
check-missing-features-in-docs	28s
markdownlint	1m 3s
check-compiles	0s
check-compiles-no-std	0s
build-wasm	0s
build-wasm-atomics	0s
msrv	0s
	41m 14s

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