add `scale_fill_continuous` example

[isabelizimm]

Here is a small example for scale_fill_continuous()!

[has2k1]

Great. Let's touch it up and have it contribute to the gallery.

1. Show a sneak-pick of the data faithfuld.head()
2. Avoid the variable (v) so that all the plotting code is visible in the executed cell.
3. Create a header (anywhere above the cell that creates the gallery image) that will be the title of the plot in the gallery. e.g. Annotated Heatmap
4. For the output that appears in the gallery, we can use any of the other perceptually uniform colormaps. Probably plasma or cividis, the others may have so much black! And, use coord_cartesian(expand=False) to crop out the surrounding space.
5. Finally, to get it in the tiles section of the gallery, add this # Gallery, tiles comment to the top of the code cell.