links to images on subpages

@cqt2596 see, e.g., https://www3-dev.tuhh.de/e-10/pontifex/docs/chapter0/000/

Images are linked to

https://www3-dev.tuhh.de/e-10/pontifex/docs/chapter0/pontifex.png

instead of

https://www3-dev.tuhh.de/e-10/pontifex/pontifex.png