Fixes #7636: Add Katello Disconnected user guide.

This may be an issue w/ the tool vs procedures; however, this step fails on rhel6 install:
```
# katello-disconnected setup --oauth-key=katello --oauth-secret=3C0nDblLqWS33fCWJExhAwazAExaRZdS
/opt/rh/ruby193/root/usr/share/rubygems/rubygems/custom_require.rb:36:in `require': cannot load such file -- /root/lib/foreman (LoadError)
	from /opt/rh/ruby193/root/usr/share/rubygems/rubygems/custom_require.rb:36:in `require'
	from /opt/rh/ruby193/root/usr/share/gems/gems/katello-2.0.0/lib/katello.rb:31:in `<top (required)>'
	from /opt/rh/ruby193/root/usr/share/rubygems/rubygems/custom_require.rb:60:in `require'
	from /opt/rh/ruby193/root/usr/share/rubygems/rubygems/custom_require.rb:60:in `rescue in require'
	from /opt/rh/ruby193/root/usr/share/rubygems/rubygems/custom_require.rb:35:in `require'
	from /usr/bin/katello-disconnected:22:in `<main>'
```