site: Add simple website for the project

libvirt-java has recently gained its own dedicated URL, but as
of today we only use that to host the javadoc API reference
under a subdirectory and navigating to the top-level leads to
a GitLab 404 page.

Prepare a very basic website for the project. The contents are
the same as the current https://libvirt.org/java.html

 page,
just reorganized a bit; the HTML, CSS and graphics are taken
from the libvirt-php website.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>