consider removing the bottom pointers on the rendered pages

[jkrick]

These currently have a bug in the headers; while this remains we can just remove them.
Check that the bug goes away after PR #220

These were meant as though going through a book, wanting to read one chapter that follows from another chapter, but that is not how we expect people to read through the notebooks.

[bsipocz]

I label it as a bug as the current behaviour is buggy; and is reported upstream here: jupyter-book/mystmd#2589