Include plot in introductory information to illustrate concept

[sadielbartholomew]

After #171 updated the plot from POC form, it is nice to showcase it in the introductory information, with a brief note about how the optimal window is where the area under the curve is minimised, as a neat visual demonstration of the overall concept of carbon intensity minimisation which is the key concept for CATS. Namely:

• include in README;
• include in the 'quickstart' page of the docs (here the 'intro' is more about a summary and background, rather than any nitty-gritty, so best in 'quickstart' instead IMO).

Also moves the GIF to the docs.source/_static directory, updating any links to that path (just two), since all other images and media are there, as with the example plot which I generated and committed. Alternatively we could have a root-level 'media' directory, but with minimal media I don't think this is necessary.