Localize the titles and the sections

[fflorent]

Whereas the content of the website is localized, the menu of the left are not.

[fflorent]

Idea:

• [ ] Use Yaml style meta-data in documents to describe the title, remove the titles from the nav config item;
• [ ] Extract the section names from the nav config item and put them in some yaml file;
• [ ] adapt ./docs.py new-lang so the English sections names are copied to the new language and can be translated by Git Localize.