Split content and organisation of the tutorial

[ocaisa]

At the moment it is very hard not to make a complete copy of older material so that you can create a new tutorial (since you will likely want to do things like shuffle things around and remove or add content).

With https://github.com/mondeja/mkdocs-include-markdown-plugin we could separate out the main content from the links that we provide at the bottom of each page. This gives you the freedom to restructure things while also allowing you to edit existing content (by using symlinks) rather than making entirely new files (which are a PITA to review in a PR as a result). When we archive a lesson we can break the links.