doc: Expand upon protocol versions and extensions

doc: Expand upon protocol versions and extensions

First, split the Protocol Versions table in two, and lead with the list
of versions that are supported today. Reserved and unsupported version
numbers go into the second table.

Second, in anticipation of a new (reserved) protocol extension, document
the extension negotiation process alongside version negotiation, and add
the corresponding tables for future extension parameter registrations.

Reviewed-by: Jelte Fennema-Nio <postgres@jeltef.nl>
Reviewed-by: David G. Johnston <david.g.johnston@gmail.com>
Discussion: https://postgr.es/m/DDPR5BPWH1RJ.1LWAK6QAURVAY%40jeltef.nl