docs: introduce rst2html as a mandatory tool for building docs

docs: introduce rst2html as a mandatory tool for building docs

The rst2html tool is provided by python docutils, and as the name
suggests, it converts RST documents into HTML.

Basic rules are added for integrating RST docs into the website
build process.

This enables us to start writing docs on our website in RST format
instead of HTML, without changing the rest of our website templating
system away from XSLT yet.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>