Update docs to show users to use `CatalogMetadata` API instead of `Packages` and `Bundles`

[anik120]

#342 should be followed up with a docs change

[joelanford]

I updated docs in the catalogd repo as part of https://github.com/operator-framework/catalogd/pull/149, so that may be the best place to reference what updates need to be made here.

[ankitathomas]

Done as of https://github.com/operator-framework/operator-controller/pull/694