📖 docs: add ClusterClass CRD relationships diagram

[Kausheel]

What this PR does / why we need it:

The change adds a new diagram to show the ClusterClass's relationships with other CRDs, hopefully making it easier to understand how a ClusterClass fits in with the other resources.

I've tested the change with make serve-book to confirm the image sizing/alignment looks correct.

Which issue(s) this PR fixes:

Fixes #8558

/area documentation

[k8s-ci-robot]

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[k8s-ci-robot]

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[sbueringer]

Thx!

How was the image created and how can we update/maintain it going forward?