Improve support for API categories

**What would you like to be added**:
- Explain API categories better within the built in `kubectl` documentation
- Signpost people to `kubectl api-resources --categories foo`
- Make categories more useful to people defining custom APIs

**Why is this needed**:
Per issues https://github.com/kubernetes/kubectl/issues/151 and https://github.com/kubernetes/website/issues/56279, the _category_ concept is not well documented. Surfacing it via the `kubectl` built in help would enable users to understand more.

That same content is automatically used to generate parts of the Kubernetes documentation, so some improvements need to happen upstream in [k/k](https://github.com/kubernetes/kubernetes).

/kind feature