doc: Improve documentation discoverability for pgoutput.

doc: Improve documentation discoverability for pgoutput.

Previously, the documentation for pgoutput was located in the section on
the logical streaming replication protocol, and there was no index entry
for it. As a result, users had difficulty finding information about pgoutput.

This commit moves the pgoutput documentation under the logical decoding
section and adds an index entry, making it easier for users to locate and
access this information.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Euler Taveira <euler@eulerto.com>
Discussion: https://postgr.es/m/CAHGQGwFJTbygdhhjR_iP4Oem=Lo1xsptWWOq825uoW+hG_Lfnw@mail.gmail.com