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)

Closes #1236
doc: Put embedding documentation into the guide
doc: Add 'make upload-doc' target

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