docs(cropping): switch to the Xenium cells dataset, show the zoom explicitly

Real tissue instead of blobs; box + connector lines mark the crop in
section 1 and the thumbnail; contrast section uses a dim window so the
window-vs-full stretch is visible.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011JMbY8asahxaytthjvCKDv

Author: timtreis