Add HTML support

[mgielda]

Would allow us to test e.g. https://risc-v-getting-started-guide.readthedocs.io/en/latest/zephyr-qemu.html

(although these actually come as scripts so it's perhaps not the best example)

anyhoo, the ability to test arbitrary HTML code snippets would be pretty rad

[mgielda]

We could focus (for now) on how Sphinx renders things. So basically unwrap all spans, and you're done.

Tabs like here: https://risc-v-getting-started-guide.readthedocs.io/en/latest/linux-qemu.html are a difficulty, but if you properly extract the snippets, I don't think it really is.

Also, just came to my mind, in RST, naming the snippets that come in tabs could solve the problem, and the tabbed content is mostly generated anyway.