doc: Make logical replication examples executable in bulk and legal sgml.

While responding to a "our documentation is buggy" complaint I got annoyed
in my attempt to reproduce the behavior by having to surgically copy
line-by-line the DDL and DML code involved. Let's strive for a more
copy-paste friendly example setup. No prompts and no interspersed command
tags (they are ok if the script is one block and the output is another).

In passing, the examples use < 5 and >= 5 in expressions - which in theory
are supposed to require the use of &lt; and &gt; entities...or enclosure in
a CDATA construct. The latter seems preferable.

David J.