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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[negz]

Two nits but overall LGTM. Thank you!

[negz]

This isn't needed in P&T.

[negz]

Suggested change

	title: App Compositions Quickstart
	title: App Quickstart

Nit: The other titles are e.g. "AWS Quickstart", "GCP Quickstart" - without "Composition".

"App Model Quickstart" might be good too.

[haarchri]

@cwilhit thank you for the work you put into this PR.

One important bit of context that has changed since this PR was opened:
Starting with Crossplane v2, composing arbitrary Kubernetes resources is supported out of the box. Because of that, the original framing of this PR around using provider-kubernetes for building the application model overlaps with functionality that is now part of the core composition experience.

That said, there is still a useful angle here. Some features (for example things like managementPolicies) are not supported in the same way when working with arbitrary resources.
If there is a need to document those gaps, tradeoffs, or specific patterns with provider-kubernetes, that would be great content, but it would fit better as a guide rather than core reference documentation.

So if you would like to continue this work, we would be happy to review a refreshed PR that:

• Is based on the current Crossplane v2 docs structure
• Reframes this as a guide (e.g., when and why to use provider-kubernetes with arbitrary resources)

For now, we will close this PR since it no longer aligns with the current docs state, but we would genuinely welcome a revised version.
Thanks again for the contribution and the effort you have put into it 🙌