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

Fujii Masao <fujii@postgresql.org>

Commit: 92f9468657f0916ce8589e13d5ebda60c7973c31
Author: Fujii Masao <fujii@postgresql.org>
Date: 2020-05-29T08:14:33Z
Releases: 13.0
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

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