docs: rewrite content on front page to be more useful

docs: rewrite content on front page to be more useful

The front page contains a bunch of content that is either
outdated (eg links to libvirt qpid) or not particularly
useful (links to vendor hypervisor sites).

This rewrites it to be more directly useful to visitors,
providing a updated intro to libvirt project, direct
links to key pieces of website content (new contributors
guide, security process, bug reporting, XML formats), and
feed showing recent 5 blog posts on the virt tools planet.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>