Restructure the documentation following the diataxis principles

I have moved all pages across the 4 categories of documentation. Sometimes I had to split content over different pages. To not overload the table of content, I have regrouped some pages below sub-headings for specific users.

With tutorials and explanation pages now clearly oriented to those who are new and learning, it makes for a smoother experience for newcomers.

This also led me to identify many gaps in our documentation and I have thus added "todo" directives so that we are aware of what's missing.

Relates to #163 (closed)