Faudra remplir ça les amis!

TODO: document the packages needed for Sflphone-gtk

Structure update (page_build, page_discuss, Build.txt).
Update Build.txt

Update to Build notes, update to Downloads.
Fix AsciiDoc macros.

Fixed the arrow in the content/images too.

Removed old screenshots.

We'll delete them right after, just to save them in the repos.

Moved images from doc/images to www/images since they are backend specific, and
need not be sitted beside the actual documentation (the 'NOTE' icon might
not be taken from the same place when rendered to a PDF later).

That way, we'll be able to use those icons elsewhere on the website, without
fetching them from the repository. They aren't going to be updated, also,
and thus don't need versioning very much.

Tiny fix for Build.txt