doc: no OS / browser version mentionned

[FabriceN]

In the browser menu as well as in the general site description, it is said nowhere what is the OS used (for desktops), nor the browser version, and some HW details (e.g. type of graphics card or emulation).

Concerning JS and webGL, for instance, these informations are quite important (not the same perfs, not the same bugs).

thanks !