Provide Administering Foreman guide for "foreman-el" and "foreman-deb"

[melcorr]

As part of #396, we and the latest discussion there, we need to stream the admin guide to generate versions for

• Satellite (the status quo but ensure nothing leaks that shouldn't etc)
• Foreman-EL
• Katello
• Debian users
• (I know orcharhino work has already been done, maybe more is needed)

This was inspired by https://github.com/theforeman/theforeman.org/pull/1879#discussion_r690316318

I (@melcorr) will work on the foreman-el and katello scenarios.