Add a quickstart for composing Kubernetes resources (apps) with Crossplane

[cwilhit]

This PR adds a guide to the main branch of the docs for how to compose Kubernetes objects (for app scenarios) using provider-kubernetes. There's an implementation with both patch-and-transform and KCL.

This guide assumes the user completes the prior AWS/Azure/GCP quickstart first, so it glosses over explanations of core Crossplane concepts.

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