adding documentation

[choldgraf]

This adds some documentation with Sphinx - it re-purposes the pre-existing docs for this. It also re-uses the README.md file for the main repository as the index.md for the docs, though we can always change that around later if we wish.

What do you think @chrisjsewell ? If you're +1 then I'll also add this to RTD

[chrisjsewell]

sounds good, do you want to add the circleci setup (+github action) why you're at it

[choldgraf]

sure thing, will get to it in a bit

[choldgraf]

it works @chrisjsewell !

[chrisjsewell]

looks good to me 😄

[choldgraf]

woot!

[chrisjsewell]

@choldgraf did you add to RTD, or do you want me to?

[choldgraf]

oops good catch, I just did it: https://markdown-it-py.readthedocs.io/en/latest/