Make pgoutput documentation easier to find

Hi,

The current documentation for pgoutput is buried in the logical streaming
replication protocol section (in protocol.sgml), and there's no index entry
for it. This makes it hard to discover and access, for example, when trying
to look up the options it supports.

I've often struggled to locate this information myself, so I'd like to
propose moving the pgoutput documentation to the logical decoding section
and adding an index entry. The attached patch does that. I think this change
will make it much easier for users to find the relevant details.

Thoughts?

Regards,

--
Fujii Masao