doc: index items for pg_stat_progress_xxx views

Hi,

The index items for pg_stat_progress_xxx views point to the
"Viewing Statistics" section, but not to the dedicated section
(e.g., "ANALYZE Progress Reporting") for each view. IMO this is
very inconvenient when finding the section describing each
pg_stat_progress_xxx view, from the index. So what about adding
new pointer to the section for each view in the index?
Patch attached.

BTW, other stats views have both pointers in the index.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION