Unify /docs to use markdown exclusively

[openscript]

Description:

How about unifying the format in the docs folder to .md Markdown? Currently there are .rst and .md files.

[neilisfragile]

Would happily accept a PR, though could be fiddly in places.

[dstipp]

Hi folks - I took a first pass at a few of the documents. Let me know if that's what you were thinking and I can continue or abandon ship.

[richvdh]

@dstipp: it looks great at a glance. Would you be able to make a Pull Request for your changes, so that it's easier for us to review?

It's probably best not to wait for all of the documents to be done: it's much easier for us to handle smaller PRs, and we can get value from your work sooner that way.