Promote CompositionRevisions to GA (v1)

### What's Missing?

https://docs.crossplane.io/knowledge-base/guides/composition-revisions/

The content on Composition Revisions is currently a guide, separate from content that is versioned alongside Crossplane. I'd like to update this content to reflect that Composition Revisions is GA (i.e. v1) in Crossplane v1.12 but I am not sure how to go about that.

I think we have this as a 'guide' because we wanted it to be separate from the 'main' documentation flow while it was an alpha/beta feature that folks could opt into/out of. Now that it's GA you cannot opt out of it. It's always on (in 1.12).

Ideally for folks on v1.12 I'd like to merge the content of the guide into the 'main' docs - presumably into the getting started guides and/or the Composite Resources section under Concepts. It seems like the above rationale still holds for anyone on v1.11 or earlier though, and the guides section of the content is not associated with a particular Crossplane version. Perhaps we should add a marker to the guide to let folks know that the feature is GA in v1.12 per https://github.com/crossplane/docs/issues/417?