DOC: Use single column for multiple figures in examples

This overrides sphinx-gallery's CSS to make multiple images appear
individually, which AFAICT looks better for all of our examples. It
affects the following:

- examples/axes_grid1/demo_axes_rgb.py
- examples/color/colormap_reference.py
- examples/color/named_colors.py
- examples/event_handling/pick_event_demo.py
- examples/event_handling/zoom_window.py
- examples/images_contours_and_fields/barb_demo.py
- examples/images_contours_and_fields/contourf_demo.py
- examples/images_contours_and_fields/image_demo.py
- examples/images_contours_and_fields/shading_example.py
- examples/lines_bars_and_markers/fill_betweenx_demo.py
- examples/specialty_plots/advanced_hillshading.py
- examples/statistics/boxplot_demo.py
- examples/style_sheets/style_sheets_reference.py
- examples/text_labels_and_annotations/accented_text.py
- examples/text_labels_and_annotations/annotation_demo.py
- examples/ticks/tick-formatters.py
- examples/userdemo/demo_gridspec03.py
- tutorials/colors/colormaps.py
- tutorials/colors/colors.py

Author: QuLogic

doc/_static/mpl.css

@@ -237,6 +237,11 @@ div.sphx-glr-download a:hover {
     background-color: #BCD4DF;
 }
 
+/* Do not fold multiple figures in examples into two column layout. */
+img.sphx-glr-multi-img {
+    max-width: 100%;
+}
+
 table.property-table th,
 table.property-table td {
     padding: 4px 10px;