Update "positioning" in introduction

### What's Missing?

https://docs.crossplane.io/v1.11/getting-started/introduction/ has a lot of wording that reflects past messaging of the project. For example it frames Crossplane as an extension, talks about universal control planes etc. I believe this is something carried over from the old docs - e.g. https://github.com/crossplane/crossplane/tree/v1.10.2/docs.

I think we should update this to reflect the more modern "control plane framework" positioning of the project that is reflected elsewhere, for example:

* https://www.crossplane.io
* https://github.com/crossplane/crossplane/blob/master/README.md
* https://github.com/crossplane/crossplane/blob/master/CHARTER.md

To be clear, I don't think we need to hide or ignore the fact that we're an extension to Kubernetes. This is a really useful framing for folks who know Kubernetes and probably how most folks think of us. I just don't think we want to lead with it as the overarching description of what Crossplane is.