installing packages with dependencies from private registry

### What's Missing?

The challenge of installing a package from a private registry that has further dependencies (e.g. family providers) recently came up in:

* https://github.com/crossplane/crossplane/issues/5799#issuecomment-2197390101.

We have some existing docs for this scenario, but they are not quite complete:

* https://docs.crossplane.io/latest/concepts/providers/#install-a-provider-from-a-private-registry
* https://docs.crossplane.io/latest/concepts/packages/#install-a-configuration-from-a-private-registry

The provider focused page mentions this challenge, but does not provide a solution for it:
```
Configured packagePullSecrets aren’t passed to any Provider package dependencies.
```

As @turkenh suggested in https://github.com/crossplane/crossplane/issues/5799#issuecomment-2197390101, one solution is to provide package pull secrets at the Crossplane service account level instead of to individual packages. We could document this approach so it is more discoverable for folks using private registries.