Update ddl.sgml for declarative partitioning syntax

Add a section titled "Partitioned Tables" to describe what are
partitioned tables, partition, their similarities with inheritance.
The existing section on inheritance is retained for clarity.

Then add examples to the partitioning chapter that show syntax for
partitioned tables.  In fact they implement the same partitioning
scheme that is currently shown using inheritance.

Amit Langote, with additional details and explanatory text by me

Author: simonat2ndQuadrant