Document using `function-patch-and-transform`

### What's Missing?

I'd like to start gently encouraging folks to use http://github.com/crossplane-contrib/function-patch-and-transform rather than "native" P&T. Some time after v1.15 I'd like us to consider deprecating native P&T per https://github.com/crossplane/crossplane/issues/4746.

There's several pages in the docs that cover Composition that we could update, including:

https://docs.crossplane.io/master/concepts/compositions/
https://docs.crossplane.io/master/concepts/patch-and-transform/
https://docs.crossplane.io/master/getting-started/provider-aws-part-2/
https://docs.crossplane.io/master/getting-started/provider-azure-part-2/
https://docs.crossplane.io/master/getting-started/provider-gcp-part-2/