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.
Support packages extends well beyond the server plugins.  They include agent handlers (plugins) and CLI extensions as well.  So, I don't think this belongs in server.rst.