doc: Beginnings of a docbook based guide

This will contain the public documentation of cockpit, including:

 * Deployment guide (eg: Certificates, Domain auth)
 * Develaper guide (eg: embedding, writing plugins)
 * Manual pages (although obviously also available on system)

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
doc: Put embedding documentation into the guide

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
doc: Add 'make upload-doc' target

This uploads the guide to files.cockpit-project.org

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
doc: Move module documentation into the Guide

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
doc: Document how HTTP vs. HTTPS and Certificates work

Some basic pointers for people in the Guide

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
doc: Document how to configure SSO in the guide

Closes #1236
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>