docs: dedupe full-section render and fix overlapping colorbars

The default backend already draws the 167780-cell as_points panel with
datashader, so the backend section's datashader render repeated the
full-section image. Drop that render and explain the backend in prose,
referencing the panel above.

In the shared-scale two-panel crops, draw the colourbar only on the
right panel so it no longer overlaps the neighbouring axes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: timtreis