Added a workable initial page for the libvirt Application
Development Guide, giving the online viewable options +
the available download ones (pdf, epub, srpm).

Added a link to the PDF to the main Downloads page, plus
neatened the html tags throughout the page as they
were a bit of a mess.

Added --enable-compile-warnings=error to the autogen line,
as suggested by Eric Blake.

Since DV recommended keeping the build instructions distributed with the
source, move them from the old FAQ to the downloads page.

Just removing reverences to the deprecated CVS server and the old git
on et.redhat.com

* docs/downloads.html.in: but will remain for sake of old links.

* docs/downloads.html[.in]: update the secton about
the server on git.et.redhat.com based on the plans
Jim posted.

* docs/downloads.html docs/downloads.html.in: update download
  informations after switch to git
daniel

Atsushi