Adds PDF file example to the fluid "image" viewhelper documentation.
This demonstrates a capability of the image viewhelper that isn't currently
explained in the documentation.

Change-Id: Ice82c005ec66173c19d0f29efe069d23b3f46fdc
Resolves: #77348
Releases: master, 7.6
Reviewed-on: https://review.typo3.org/49319


Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>