Add infrastructure for having images in the documentation, in SVG
format.  Add two images to start with.  See the included README file
for instructions.

Author: Jürgen Purtz <juergen@purtz.de>
Author: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
Discussion: https://www.postgresql.org/message-id/flat/aaa54502-05c0-4ea5-9af8-770411a6bf4b@purtz.de