Document Crossplane XRDs we consider part of our management API

[marians]

We have XRDs (CompositeResourceDefinitions) in https://github.com/giantswarm/crossplane-gs-apis which can be considered part of our management API, just like CRDs. They should be published in (almost) the same way.

It should be possible to extend our crd-docs-generator for this task.

Open Questions

• What are the significant differences between XRD and CRD from a user's perspective
• Should this land in a separate place, or is one area for CRDs and XRDs fine?

[mproffitt]

Related to #3508

Some work on crd-docs-generator has already been done as part of giantswarm/crd-docs-generator#230

This PR cannot be merged until microerror as been replaced in it with errors as microerror has now been deprecated and should be removed from all revisited codebases