doc_layout : introduce classes for main content on a module page

-DocContentModule : main content of a module html page
-DocContentModuleSection : sections of the main content of a module html page

-fixed some comments and variable names

Signed-off-by: Maxime Leroy <maxime.leroy76@gmail.com>
nitdoc : use doc_layout on the main content of a module html page

- changed some redef methods in MClass so they return a String and be usable for doc_layout
- fixed calls to this methods everywhere

This should have taken 2 commits, but I made the use of doc_layout first
and couldn't commit it as long as it doesn't compile.

Signed-off-by: Maxime Leroy <maxime.leroy76@gmail.com>