split up underactuated.html into chapter html files

[RussTedrake]

I did not do this originally, because there are dependencies between the chapters (e.g. chapter counters, TOC, citation counter, drake examples). But i'm now dealing with more and more corner cases to support this non-standard workflow. (MathJax3 does not appear to support partial typesetting; youtube videos with display:none are still paying the start-up cost, ...)

I think it's finally time to make the switch. Proposed solution:

• create a chapters.js that defines chapters = {'intro', 'pend', 'acrobot', ...}. Somewhere I need the part logic, as well. Each individual chapter can load chapters.js, and figure out how to set the chapter counter by finding it's name in the list.
• zap the auto-generated list of drake examples in the appendix, and just point to the new examples/ directory in binder/colab. Should be able to use <jupyter>examples/</jupyter>
• refactor (and simplify) the revealChapters code into a load method that each chapter runs individually.
• for the TOC, either generate it client-side by ajax loading (yuck), or write a (python?) script to generate it and commit it manually. (could write a bazel test to check it's correctness).
• restore bibliography
• restore parts and appendix
• properly deprecate links to underactuated.html?chapter=foo
• fix mathjax newcommands
• fix drake chapter dynamic content
• restore offline mathjax workflow

[RussTedrake]

moving last ticket item to its own issue.