Add more comprehensive crate level docs for bevy_ptr #38342

Job	Run time
check-doc	7m 15s
build (windows-latest)	4m 42s
miri	6m 3s
ci	1m 30s
build (ubuntu-latest)	1m 58s
run-examples-macos-metal	4m 15s
build (macos-latest)	2m 23s
toml	1m 54s
typos	6s
check-cfg	1m 40s
check-missing-examples-in-docs	28s
check-bevy-internal-imports	3s
check-missing-features-in-docs	46s
markdownlint	1m 9s
check-compiles	2m 57s
msrv	48s
build-wasm	33s
	38m 30s

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