Add details around package management procedures (pull, push, etc)

### What's Missing?
Crossplane package reconciliation relies on layer annotations that are only applied when `crossplane xpkg push` is used to push a package into a registry. While it is physically possible to use `docker pull`, `tag`, `push` and other container-based commands to move packages around, they will not add the layer annotations required by Crossplane to efficiently cache and reconcile the packages.

There should be some specific information in the Packages section that details the fact that `docker pull`, `tag`, `push` can be used but will not add the layer annotations that Crossplane needs. Commands such as `crane pull` can be used to retrieve the Package from a registry, and `crossplane xpkg push` is currently the only "official" way to push packages to a registry with the associated annotations.

Some related Crossplane issues on the topic:

https://github.com/crossplane/crossplane/issues/5579
https://github.com/crossplane/crossplane/issues/5580