docs: Convert 'docs' index page to rst

docs: Convert 'docs' index page to rst

Along with the conversion we need to adapt the stylesheets to apply to
the new document similarly to how we do that in the knowledge base.

Note that one visible difference is that now a 'Documentation' heading
is visible on top of the page.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>