doc: Add Identity Column section under Data Definition chapter
This seems to be missing since identity column support was added. Add
the same. This gives a place to document various pieces of
information in one place that are currently distributed over several
command reference pages or just not documented (e.g., inheritance
behavior).
Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Author: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://www.postgresql.org/message-id/flat/CAExHW5uOykuTC+C6R1yDSp=o8Q83jr8xJdZxgPkxfZ1Ue5RRGg@mail.gmail.com