Fix formatting in the custom derivatives notebook

[shoyer]

Sphinx is apparently quite picky about consistent use of headers: you can't
skip a header level. We were getting warnings like "WARNING: Title level
inconsistent" in the docs build, and sub-headers weren't showing up on this
page after the first section.

[mattjj]

Hrm, I remember liking how this rendered in Colab, but not as much in Sphinx. Anyway if you think this is better then that's good enough for me!

[shoyer]

The problem is that sphinx defaults to hiding headers at the "wrong" indentation. So the online docs (which I suspect is how most users read them) don't have any subheaders. This makes them harder to read and harder to link to.

e.g.,

Before:

After -- notice the new "Numerical stability" header: