Description of your changes

Many months ago, I remember that in the initial design of Configuration to create a Crossplane package, it needs to put a package.yaml in the project folder.

Now it seems not the case anymore, and I found that the page to describe xpkg seems not up-to-date to reflect the requirements of creating a Crossplane package.

In this page it mentioned, to build a Crossplane package, it needs crossplane.yaml where the docs here says the package.yaml is required.

It causes some confusion, so this PR is trying to address this issue, and rename the package.yaml into crossplane.yaml.

I have:

• Read and followed Crossplane's contribution process.
• Run earthly +reviewable to ensure this PR is ready for review.
• Added or updated unit tests.
• Added or updated e2e tests.
• Linked a PR or a docs tracking issue to document this change.
• Added backport release-x.y labels to auto-backport this PR.

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