Add more comprehensive crate level docs for bevy_ptr #38470

Job	Run time
build (windows-latest)	5m 41s
check-cfg	1m 28s
check-bevy-internal-imports	4s
check-doc	7m 30s
build (ubuntu-latest)	2m 59s
check-missing-examples-in-docs	35s
miri	5m 51s
ci	2m 33s
run-examples-macos-metal	4m 23s
toml	1m 55s
build (macos-latest)	4m 27s
typos	4s
check-missing-features-in-docs	29s
markdownlint	1m 11s
check-compiles	3m 18s
build-wasm	1m 11s
msrv	1m 8s
	44m 47s

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