PDF exercises no longer have link to solutions

[Kerrick]

I see from #41 that the PDF used to come with links to solutions. However, the PDF seems not only solution-free, but also free of links to solutions.

FWIW I built it with yarn pdf from commit 781eebb.

[martin-henz]

The current PDF is meant to be printable in that it can be sent to the publisher as-is. Links to solutions would complicate the publishing process. The reader should refer to the online edition if they want to work with solutions.

[martin-henz]

Actually, we could link to solutions silently, just like the TOC.