Document Registering Hosts upstream

[spetrosi]

The Registering Hosts section that is available downstream is hidden upstream because of information about subscriptions that is irrelevant upstream.
From @lzap :

Technically, all of this would actually work but there's confusing amount of info about subscriptions etc. It makes sense to someone familiar with Satellite, but random upstream user would be only confused.

It is required to review the section and make it usable upstream too.

[maximiliankolb]

closing this issue: we have Registering hosts and setting up host integration for Katello. "foreman-el" and "foreman-deb" will follow for stable versions as soon as we review+provide the "Managing Hosts" guide for those build targets.