This restructures the doc as a tutorial leading newcomers to becoming experimented users.

It adds pages on:
 - how to get started with Cartographer on a new .bag
 - how Cartographer works and can be tuned step-by-step
 - how to use "extra" features of Cartographer
 - how to contribute to Cartographer

It also provides some cosmethic changes with: new titles, non-indented code blocks, various illustrations, reworked paragraphs for clarity...