docs: Add a new section and a table listing protocol versions

docs: Add a new section and a table listing protocol versions

Move the discussion on protocol versions and version negotiation to a
new "Protocol versions" section. Add a table listing all the different
protocol versions, starting from the obsolete protocol version 2, and
the PostgreSQL versions that support each.

Discussion: https://www.postgresql.org/message-id/69f53970-1d55-4165-9151-6fb524e36af9@iki.fi