Design a way to document feature gates across the project

**This is a Feature Request**

**What would you like to be added**
1. Devise how we'll document feature gates other than for core, mandatory Kubernetes components
2. Adopt that design for https://k8s.io/docs/

**Why is this needed**
We have optional components, add-ons, and tools. Many of these optional components have feature gates too, including:

- `kube-proxy`
- the [Descheduler](https://sigs.k8s.io/descheduler) add-on
- `kubeadm`
- Kueue
- the VerticalPodAutoscaler add-on
- `kubectl`

…and more!

It'll help our readers to document all the feature gates in one place, wherever they apply.
We should try to do something simple and improve it, rather than trying to do something perfect.

**Comments**
/kind feature
/language en
/triage accepted
/priority backlog
Also see these issues:
- https://github.com/kubernetes/website/issues/25645
- https://github.com/kubernetes/website/issues/55775

Start by reviewing the existing feature-gate documentation in https://k8s.io/docs/ and the related issues 25645 and 55775. Define a simple design covering optional components such as kube-proxy, kubeadm, Kueue, kubectl, and add-ons, then apply that design across the relevant documentation. Done means the design is agreed and the applicable feature gates are documented in one discoverable place.

Written by the indexing model from the issue text.