Document recent changes to render

[adamwg]

What's Missing?

crossplane/crossplane#7339 makes a fairly significant change to how crossplane render works, which brings a few new limitations and a few new features. We should document:

• How to override the Crossplane release, image, or binary to use for rendering.
• Limitations around Docker:
  • By default, render runs the Crossplane controller image under Docker, but this can be avoided with the --crossplane-binary flag.
  • Injecting or showing function pipeline context requires mounting a socket into the Crossplane docker container when using Docker mode. This does not work on Windows.