Resolve errors building documentation PDF

[jsiirola]

Fixes # .

Summary/Motivation:

The PDF version of the Sphinx documentation broke after the doc reorg; however, RTD wasn't reporting the failure. After a recent update (to RTD and/or Sphinx), RTD is now reporting the PDF failure as a failure.

This PR resolves the LaTeX errors in carious math environments throughout the documentation. It also implements a method where the HTML target retains the diataxis tabular format, but uses a linear format for the LaTeX target. In addition, the PDF target is split into two files: the main documentation (~380 pages) and the API reference (~3500 pages).

Changes proposed in this PR:

• resolve math mode errors in documentation
• rework the documentation formatting so that the PDF target is rendered linearly (and not in tabular form)
• add new Sphinx extension for making "TOC references" (so HTML and LaTeX can both have custom TOCs)
• get unicode support working for RST / docstring inputs
• remove (likely incorrect / broken) definitions for texinfo and manpage documentation targets

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