Apparently there is some problem with using file->lines to load up code inside codeblock.

Instead of storing the example program in example-cover.rkt and trying to sideload it into overview.scrbl, probably better to actually keep it inside overview.scrbl and run it in an examples evaluator. Thus the example-cover.pdf file will get updated every time the docs are rebuilt.

The bootstrapping process for the docs thus becomes:

0. touch scribblings/example-cover.png (to prevent errors the very first time you build scribble docs)
1. Build scribble docs, thus creating my-book.pdf and example-cover.pdf
2. make png, thus updating example-cover.png
3. git commit -m 'Update example cover png'