docs: add a Scalebars tutorial (#12)

* docs: add a Scalebars tutorial

A focused tutorial on drawing physical scalebars via the scalebar_* arguments
of .pl.show(), on the synthetic blobs dataset:

- scalebar_dx (the one required piece) and why it's needed: SpatialData
  coordinate systems carry no unit metadata, so you supply the physical size
  of one axes-unit.
- scalebar_units and automatic magnitude scaling (um -> mm).
- styling via scalebar_params (placement, colour, box, length_fraction,
  scale_loc, fonts) - forwarded verbatim to matplotlib-scalebar.
- fixed_value for an exact bar length.
- cross-links the Visium example for a real-data figure.

Wired into the gallery: card in tutorials/index.md (inside the gallery-cards
markers, root-relative paths) + toctree entry + thumbnail. Addresses part of
scverse/spatialdata-plot#463.

* docs(scalebars): fix white-on-white bar in the fonts and fixed-length cells

Those two cells set color="white" but relied on matplotlib-scalebar's default
white box, so the bar landed on the box and vanished. Give them a translucent
dark box (box_color/box_alpha), and add a note to the appearance section that
white bars on a dark image need a dark box or frameon=False.

* docs(scalebars): tighten prose, fix thumbnail, correct asset-path docs

- Prose pass on the scalebar tutorial markdown cells.
- Regenerate the gallery thumbnail from the notebook's final plot: no
  title/axes, landscape ~1.11 ratio to match the sibling cards.
- CONTRIBUTING: reference assets by the submodule-mount paths (absolute
  /notebooks/... for index.md cards, relative ../ from notebook cells).

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

* docs(scalebars): add rotation + label section, sync thumbnail

- New §5 demonstrating a vertical bar (`rotation`) and a custom bar title
  (`label`/`label_loc`); old wrap-up renumbered to §6.
- Regenerate the gallery thumbnail from this final plot: vertical 100 µm
  bar with a "scale" label, landscape ~1.11 to match the sibling cards.
- Re-executed end-to-end so committed outputs are current.

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

---------

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

Author: timtreis

CONTRIBUTING.md

@@ -36,10 +36,14 @@ pre-commit install
    toctree.
 
 6. Place the gallery thumbnail and any other media (GIFs, screenshots, static
-   PNGs referenced from the notebook) under `_static/img/`. Reference them from
-   `index.md` and notebook markdown cells with relative paths
-   (e.g. `:img-top: ../_static/img/<slug>.png` from inside `examples/` or
-   `tutorials/`, and `![…](../_static/img/<slug>.gif)` from a notebook cell).
+   PNGs referenced from the notebook) under `_static/img/`. This repo is mounted
+   as a git submodule at `docs/notebooks/` in the `spatialdata-plot` Sphinx
+   build, so paths resolve against that mount:
+   - **Gallery cards in `index.md`** — use the absolute source-root path:
+     `:img-top: /notebooks/_static/img/<slug>.png`.
+   - **Notebook markdown cells** — use a path relative to the notebook:
+     `![…](../_static/img/<slug>.gif)`.
+
    Do **not** drop assets next to the notebook itself.
 
 7. Open a PR. CI will:

_static/img/scalebars.png

@@ -35,6 +35,15 @@ selection for images, and the `datashader` backend for large collections
 of shapes and points.
 :::
 
+:::{grid-item-card} Scalebars
+:link: /notebooks/tutorials/scalebars
+:link-type: doc
+:img-top: /notebooks/_static/img/scalebars.png
+
+Add a physical scalebar with `scalebar_dx`, choose units, and style
+placement, colour, length and fonts through `scalebar_params`.
+:::
+
 ::::
 <!-- gallery-cards-end -->
 
@@ -45,4 +54,5 @@ of shapes and points.
 getting_started
 color_and_palette
 performance
+scalebars
 ```