Aggregation docs don't explain webhooks

[rcorre]

This is a...

• Feature Request
• Bug Report

Problem:

The aggregation docs mention "Webooks" several times when comparing CRDs to API server aggregation. For example, the table comparing the two has these rows:

Feature	Description	CRDs
Validation	Help users prevent errors and allow you to evolve your API independently of your clients. These features are most useful when there are many clients who can’t all update at the same time.	Yes. Most validation can be specified in the CRD using OpenAPI v3.0 validation. Any other validations supported by addition of a Validating Webhook
Custom Business Logic	Perform arbitrary checks or actions when creating, reading, updating or deleting an object	Yes, using Webhooks.

So how do I actually use "webhooks" to implement validation or custom business logic? There are no links here, so I searched and found the docs on Webhook Mode, which seems to be about auth, and admission controllers, which only discusses built-in controllers. I'm not even clear if I can create my own admission controller:

The controllers consist of the list below,

Proposed Solution:

Include a section or separate page on how to use webhooks for CRD business logic and validation, or at least make and reference to "Webhooks" a link to the page explaining this kubernetes feature, since neither of the pages I found seem to be relevant.

Page to Update:
https://kubernetes.io/docs/concepts/extend-kubernetes/api-extension/custom-resources/#api-server-aggregation

[CaoShuFeng]

Hi.

#9676