Add docs explaining SIMP architectural decisions

There should be a place in the documentation where some architecture or infrastructure decisions that may be abnormal are explained, like:

• How SIMP assumes an admin would manage packages

• How SIMP assumes people want to provision their systems (kickstart)

• How SIMP assumes an admin would want to distribute large files (rsync)

The security_conop guide is thorough and explains a lot of these types of assumptions, but we should further explain things that would confuse a new admin, even if they have set up their own infrastructure before.