doc: Add section for temporal tables

doc: Add section for temporal tables

This section introduces temporal tables, with a focus on Application
Time (which we support) and only a brief mention of System Time (which
we don't).  It covers temporal primary keys, unique constraints, and
temporal foreign keys.  We will document temporal update/delete and
periods as we add those features.

This commit also adds glossary entries for temporal table, application
time, and system time.

Author: Paul A. Jungwirth <pj@illuminatedcomputing.com>
Discussion: https://www.postgresql.org/message-id/flat/ec498c3d-5f2b-48ec-b989-5561c8aa2024@illuminatedcomputing.com