doc: convert to Sphinx documentation at libceed.readthedocs.io

Some technical notes about the expanded docs/process:

* Doxygen is now restricted to API documentation. Sphinx docs are
incomplete

doc: convert to Sphinx documentation at libceed.readthedocs.io

Some technical notes about the expanded docs/process:

* Doxygen is now restricted to API documentation. Sphinx docs are
incomplete surrogates due to not hyperlinking code listings, though
we hope the Doxygen site can be removed eventually.

* PDF output uses XeTeX (unicode and OTF fonts) with TeX Gyre Pagella
and DejaVu Sans Mono. You may need to install these fonts in order
to build the PDF users manual.

* Obtaining the author list from Git has not worked on
ReadTheDocs (`git shortlog` is empty even though `git log` is not),
so we automatically generate it on developer machines with `make
doc`. If the author information is incorrect/mangled, edit .mailmap
to fix the mapping.

* Math semantics are based on the description here
https://github.com/sphinx-doc/sphinx/issues/3785

Sphinx chooses one of two setups for the maths.

* If there is a single equation, then we wrap the maths in
\begin{equation*} (or equation).

* If there are multiple equations (as determined by the presence of
an empty line), then we wrap the maths first in \begin{aligned}
and then in \begin{align*} (or align).

Co-authored-by: Jeremy L. Thompson <jeremy.thompson@colorado.edu>
Co-authored-by: Jed Brown <jed@jedbrown.org>

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