Add Colour and palettes card to gallery (#663)

Author: timtreis

docs/_static/css/custom.css

@@ -2,3 +2,15 @@
 div.cell_output table.dataframe {
     font-size: 0.8em;
 }
+
+/* Gallery cards: normalise thumbnail height so cards line up regardless
+   of the underlying image aspect ratio. Image is contained (not cropped)
+   on a white background, centred in the slot. */
+.sd-card-img-top {
+    height: 240px;
+    width: 100%;
+    object-fit: contain;
+    background: #ffffff;
+    padding: 8px;
+    box-sizing: border-box;
+}

docs/_static/gallery/color_and_palette.png

@@ -24,6 +24,16 @@ The fluent `.pl` API, layering, and styling on the in-memory `blobs`
 dataset. Ideal first read.
 :::
 
+:::{grid-item-card} Colour and palettes
+:link: notebooks/tutorials/color_and_palette
+:link-type: doc
+:img-top: _static/gallery/color_and_palette.png
+
+How `color=` resolves, the v0.3.0 `groups` behaviour, and building
+perceptually well-spaced or colourblind-safe palettes with
+`make_palette` and `make_palette_from_data`.
+:::
+
 ::::
 
 ## Examples