Use the class HTML attribute to better style the documentation

[omichel]

Using the class HTML attribute in the documentation will help us avoiding conflicts with the CSS of Cyberbotics' web site. The name of the class could be webots-doc. We should split the current main.css file into two files: main.css would remain the main CSS file of Cyberbotics' web site whereas webots-doc.css would contain only the class specific CSS for the Webots documentation. Only the webots-doc.css stylesheet file should be distributed with Webots.