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>

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

@@ -83,7 +83,7 @@
    "source": [
     "## 1. The one required piece: `scalebar_dx`\n",
     "\n",
-    "`scalebar_dx` is the physical size of **one axes-unit**. The `blobs` image is indexed in pixels, so one axes-unit is one pixel; at our assumed resolution that is 0.5 µm. Pass that as `scalebar_dx`, and the unit it is measured in as `scalebar_units`. Note that `scalebar_dx` should be a float."
+    "`scalebar_dx` is the physical size of **one axes-unit**. The `blobs` image is indexed in pixels, so one axes-unit is one pixel; at our assumed resolution that is 0.5 µm. Pass that as `scalebar_dx` (a float), and the unit it is measured in as `scalebar_units`."
    ]
   },
   {
@@ -139,7 +139,7 @@
    "source": [
     "## 2. Units and auto-scaling\n",
     "\n",
-    "`scalebar_units` is simply the unit your `scalebar_dx` is expressed in. matplotlib-scalebar then picks a human-friendly magnitude automatically: a bar spanning thousands of µm is shown in mm. The same image at a coarse 50 µm/pixel gives a bar labelled in mm:"
+    "`scalebar_units` is simply the unit your `scalebar_dx` is expressed in. In the background, it uses [matplotlib-scalebar](https://pypi.org/project/matplotlib-scalebar/), which picks a human-friendly magnitude automatically — a bar spanning thousands of µm is relabelled in mm. Here the same image at a coarse 50 µm/pixel crosses that threshold:"
    ]
   },
   {
@@ -181,7 +181,7 @@
    "source": [
     "## 3. Placement and appearance with `scalebar_params`\n",
     "\n",
-    "`scalebar_params` is a dict forwarded **verbatim** to matplotlib-scalebar's `ScaleBar`, so every option it supports is available. The ones you will reach for most:\n",
+    "`scalebar_params` is a dict forwarded to matplotlib-scalebar's `ScaleBar`, so every option it supports is available. The ones you will reach for most:\n",
     "\n",
     "- `location` — which corner (`\"lower right\"`, `\"upper left\"`, …).\n",
     "- `color` — bar and text colour (use white on dark images).\n",
@@ -348,10 +348,34 @@
   },
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 1,
    "id": "8737ae6c",
    "metadata": {},
-   "outputs": [],
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Python implementation: CPython\n",
+      "Python version       : 3.14.4\n",
+      "IPython version      : 9.13.0\n",
+      "\n",
+      "spatialdata        : 0.7.3\n",
+      "spatialdata_plot   : 0.4.0\n",
+      "matplotlib         : 3.10.9\n",
+      "matplotlib_scalebar: 0.9.0\n",
+      "\n",
+      "Compiler    : Clang 20.1.8 \n",
+      "OS          : Darwin\n",
+      "Release     : 25.2.0\n",
+      "Machine     : arm64\n",
+      "Processor   : arm\n",
+      "CPU cores   : 8\n",
+      "Architecture: 64bit\n",
+      "\n"
+     ]
+    }
+   ],
    "source": [
     "# ruff: noqa: F401, F811, I001, E402\n",
     "# fmt: off\n",
@@ -365,9 +389,9 @@
  ],
  "metadata": {
   "kernelspec": {
-   "display_name": "Python 3",
+   "display_name": "sdata-plot-notebooks",
    "language": "python",
-   "name": "python3"
+   "name": "sdata-plot-notebooks"
   },
   "language_info": {
    "codemirror_mode": {
@@ -379,9 +403,9 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.13.7"
+   "version": "3.14.4"
   }
  },
  "nbformat": 4,
  "nbformat_minor": 5
-}
+}