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