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.
The services.rst page was intended to document all of the platform services not only the sever-side services.  It also includes the agent.  It seemed useful for users to look in one place to see all of the services.  That said, we could put this back into server.rst and add a consumer.rst to better document consumer-side components.  This could include the consumer (agent) service.  Best I can tell, the consumer components of pulp are not centrally documented but instead talked about here-and-there.