XP-007 Add docs

[rchaput]

Documentation should be added in a docs/ folder at the root of the project. It should contain:

• the code documentation (extracted from the Python docstrings)
• a "user manual" on how to use the application
• a "dev manual" on how to contribute, how to delve into the source code, what is the goal of each module,...

The documentation should be set up as Github Pages for easy access.