Add diagram showing admission control phases

[lmktfy]

The key aim of this PR is to add a diagram [preview] to http://k8s.io/docs/reference/access-authn-authz/admission-controllers/

There was some early work in #43836 which is good but doesn't render. Then @sftim - my previous GitHub identity - moved this forward as #48131

It's quite a detailed diagram; I actually had an even more complex one but simplified it to this. I can think of changes to make (eg: skip the HTTP interactions; make two diagrams that work like a slideshow) but I think that all the complexity on one screen is OK.

[k8s-ci-robot]

PR needs rebase.

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