Make sure all PDF image links work

[BartMassey]

Apparently the "export as PDF" button is having trouble with Wikipedia/Wikimedia image links right now? Needs looking at.

[iampi31415]

The issue seems on the page, then get's passed onto the PDF as well.

Most images do show correctly, but not the SVG in I2C.

For current PR, this is what I see:

It could be related to the Github-hosting only. I did not try locally.

[iampi31415]

This can be solved changing the src to https://upload.wikimedia.org/wikipedia/commons/0/04/I2C_controller-target.svg instead of the current one.

It also needs class="white_bg".