pgsql: doc: Update the layout of "Viewing Statistics" section.

doc: Update the layout of "Viewing Statistics" section.

This commit updates the "Viewing Statistics" section more like
the existing catalogs chapter.

- Change its layout so that an introductory paragrap is put above
   the table for each statistics view. Previously the explanations
   were below the tables.

- Separate each view to different section and add index terms for them.

Author: Fujii Masao
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/6f8a482c-b3fa-4ed9-21c3-6d222a2cb87d@oss.nttdata.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/92f9468657f0916ce8589e13d5ebda60c7973c31

Modified Files
--------------
doc/src/sgml/catalogs.sgml            |   5 +-
doc/src/sgml/config.sgml              |  19 +-
doc/src/sgml/high-availability.sgml   |   6 +-
doc/src/sgml/logical-replication.sgml |   3 +-
doc/src/sgml/logicaldecoding.sgml     |   3 +-
doc/src/sgml/monitoring.sgml          | 375 ++++++++++++++++++++++++----------
doc/src/sgml/ref/initdb.sgml          |   3 +-
doc/src/sgml/ref/pg_basebackup.sgml   |   3 +-
doc/src/sgml/sslinfo.sgml             |   3 +-
9 files changed, 300 insertions(+), 120 deletions(-)