Introduction/README: replace cf-plot lone static image with gallery

In our 'Introduction' docs page and equivalent README summary, we currently we advertise cf-plot as shown in the screenshot below, but providing just one image to showcase the library under-sells it, so I would like to update the lone image with a small gallery of a diverse set of possible plots made with cf-plot, very similar to the one in the present documentation but possibly condensed a bit so as not to take up too much space (don't want to bloat the README or people won't read it, at least all the way through). Ideally this could be a scrolling gallery, but I am not sure if this is still possible with Sphinx.

And whilst we do link directly to the cf-plot gallery but I think it would be nicer to ideally embed it, or if not provide a screenshot of a wider view of the full gallery showing multiple plot types possible, or provide a similar thing, directly in our Introduction/README in place of the single image we have now.

Current view of section advertising cf-plot library