docs: Merge separate chapters on built-in index AMs into one.

docs: Merge separate chapters on built-in index AMs into one.

The documentation index is getting very long, which makes it hard
to find things. Since these chapters are all very similar in structure
and content, merging them is a natural way of reducing the size of
the toplevel index.

Rather than actually combining all of the SGML into a single file,
keep one file per <sect1>, and add a glue file that includes all
of them.

Discussion: http://postgr.es/m/CA+Tgmob7_uoYuS2=rVwpVXaRwP-UXz+++saYTC-BCZ42QzSNKQ@mail.gmail.com