Docs: make documentation using GitHub Pages

[ma-ilsi]

Background

Where do we currently stand in documentation efforts? What is the problem?

We have no documentation. Without documentation, end users won't know how to use Sophie. Contributors won't be able to contribute efficiently, either.

Proposal

Documentation fixes, recommendations, and plans

GitHub Pages

After some internal (in my head, since no one else is here right now) debating between choosing GitHub Wiki on this repository or GitHub Pages, I've decided to use GitHub Pages. The Wiki lacks customisation and that makes it hard to enjoy mantaining. I also think I'll learn more using Pages and it opens up more opportunities for people to help (workflows, etc). Probably Jekyll, less time, supported out of the box (but suggestions are welcome).

Diátaxis

We'll be following the diátaxis methodology of maintaining documentation (with a few exceptions where needed).
This gives some clear rules on how to write and how to structure the docs. It's also easy to point contributors to the source so they know what our documentation is founded upon.