pgsql: doc: Add Identity Column section under Data Definition chapter

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a37bb7c13995b834095d9d064cad1023a6f99b10

Modified Files
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doc/src/sgml/datatype.sgml |   2 +-
doc/src/sgml/ddl.sgml      | 110 +++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 111 insertions(+), 1 deletion(-)