1145320 - document running pulp-manage-db after installation.

https://bugzilla.redhat.com/show_bug.cgi?id=1145320

We reference the platform services in a number of places so it seemed a good idea to make it a separate document and just reference it.
Instead of documenting each of these and how to restart them, why don't we document the names and purposes of each service and then separately document how to start, stop, status, and restart a service in general? This page gets pretty repetitive with all the examples of restarting when the only thing that's different is the service name.