pgsql: Doc: Add the new section "Logical Replication Failover".

Doc: Add the new section "Logical Replication Failover".

This aids the users to ensure that the failover marked slots are synced
to the standby and subscribers can continue replication even when the
publisher node goes down.

Author: Hou Zhijie, Shveta Malik, Amit Kapila
Reviewed-by: Peter Smith, Bertrand Drouvot
Discussion: https://postgr.es/m/OS0PR01MB57164D6F53FB4F6AD29AD9C594FB2@OS0PR01MB5716.jpnprd01.prod.outlook.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b560a98a17aebf3fabbb8dec89f506b52e61c26b

Modified Files
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doc/src/sgml/high-availability.sgml   |  9 ++++
doc/src/sgml/logical-replication.sgml | 94 +++++++++++++++++++++++++++++++++++
2 files changed, 103 insertions(+)