DOC: restore step by step compilation for documentation

The step by step documentation for not advanced users was removed.

The current version is not oriented towards helping the user since !36 (merged).

Several information such as how to access the sources, navigate inside, and configure sphinx are now missing.

This may seem trivial to a developer but was very useful during previous training.

We have to keep in mind that we have users from very different backgrounds and keeping explanations as complete and simple as possible is essential.