Support GitHub Light/Dark Image Switcher

[daveverwer]

Discussed in #1397

^{Originally posted by Sherlouk November 25, 2021}
GitHub have recently announced a new feature which allows for markdown documents (such as the READMEs we render) to support images which change based on light/dark mode.

https://twitter.com/ashtom/status/1463554157932023808

![GitHub-Mark-Light](https://user-images.githubusercontent.com/3369400/139447912-e0f43f33-6d9f-45f8-be46-2df5bbc91289.png#gh-dark-mode-only)![GitHub-Mark-Dark](https://user-images.githubusercontent.com/3369400/139448065-39a229ba-4b06-434b-bc67-616e2ed80c8f.png#gh-light-mode-only)

https://docs.github.com/en/github/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#specifying-the-theme-an-image-is-shown-to

I don't know how much work it will be to handle this, or indeed what format we get it back in the HTML from GitHub (it might even support it for free!) but would be good to look into this.