docs(gallery): add "Speeding up rendering" tutorial card

Adds the gallery card + thumbnail for the new rendering-performance tutorial
and advances the docs/notebooks submodule to the commit that introduces it.

Depends on scverse/spatialdata-plot-notebooks#8 — the submodule pointer here
references that PR's head commit so the docs build/preview renders the new
tutorial. Once #8 merges, the pointer should be advanced to notebooks `main`.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: timtreis

docs/_static/gallery/performance.png

@@ -34,6 +34,16 @@ perceptually well-spaced or colourblind-safe palettes with
 `make_palette` and `make_palette_from_data`.
 :::
 
+:::{grid-item-card} Speeding up rendering
+:link: notebooks/tutorials/performance
+:link-type: doc
+:img-top: _static/gallery/performance.png
+
+Keep rendering fast on large data: automatic rasterization and scale
+selection for images, and the `datashader` backend for large collections
+of shapes and points.
+:::
+
 ::::
 
 ## Examples