Document custom asset_container_contents cache store

[ryanmitchell]

See: statamic/cms#11481

Should be pretty much the same as
https://statamic.dev/static-caching#custom-cache-store

[ryanmitchell]

Also document asset_meta cache store statamic/cms#11512

[duncanmcclean]

We could probably mention it under Assets -> Performance too.