9309: Separate PostgreSQL setup page in Install Guide.

9309: Separate PostgreSQL setup page in Install Guide.

This provides us with a few benefits:

* We have a place to discuss the different deployment options
  installers have around PostgreSQL.
* PostgreSQL setup is very distro-specific (and it's going to get
  worse when we add CentOS 7), so this can take some of that noise out
  of the Rails server install guides.
* People who want to try new things, like cloud database services,
  get a clearer separation of the install process and the database
  setup process.