Add section on certificates layout

[ehelms]

I couldn't find a spot where we laid this out in detail, so I wrote it up and am starting here to capture the information. Perhaps this best lives here, perhaps best in foreman-documentation.

There is some information, that is out of date in some spots, also collected in these locations:

• https://github.com/theforeman/foreman-installer/blob/develop/KATELLO.md
• https://github.com/theforeman/puppet-certs

Consolidating and making this better will be a useful activity that I will continue to look into. I do not want that cleanup to necessarily detract from this information which I hope can be helpful in understanding and being able to make changes in the future.

A few questions:

• Is there anything missing?
• Is there additional information that would be helpful in understanding?