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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doc: separate Doxygen groups by Public API/Backend API/Internal Functions

Helped-by: Valeria Barra <valeriabarra21@gmail.com>
Squashed-by: Jed Brown <jed@jedbrown.org>

make style

Interface - update constants to be const...ant

ElemRestriction - make strides const

Co-Authored-By: Jed Brown <jed@jedbrown.org>

ElemRestriction - add CEED_ELEMRESTRICTION_NONE so we can stop making dummy restrictions for CEED_EVAL_WEIGHT fields

Make style

Restriction - add CEED_STRIDES_BACKEND

Restriction - expand codecov

Restriction - replace identity restrictions with strided restrictions

Merge branch 'jed/ns-dmplex' [PR #458]

* jed/ns-dmplex: (73 commits)
NS: remove density_current_primitive (still work in progress)
Remove a bunch of unused variables and update doc
Update READ

Merge branch 'jed/ns-dmplex' [PR #458]

* jed/ns-dmplex: (73 commits)
NS: remove density_current_primitive (still work in progress)
Remove a bunch of unused variables and update doc
Update README: remove density_current_primitive problem option for now
Use const in VLA declaration casts to suppress icc warning also in densitycurrent_primitive
Remove a bunch of unused or double-declared variables
Update README
NS: use const in VLA casts to suppress icc warning
NS: add command-line support for specifying DC center/axis
NS: return to insulated no-slip walls instead of full-Dirichlet
NS: remove dead code for scaling VTK output back to dimensional units
NS: add support for free slip wall boundary conditions
NS density_current: remove redundant initial/boundary condition and trailing whitespace
Some changes in documentation
Update top-level README
Update examples/README
Update examples/navier-stokes/README
Update .gitignore
Update documentation
Initialize *problem pointer as NULL
NS: move nsplex.c to navierstokes.c
...

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ElemRestriction - make lmode a separate enum

ElemRestriction - remove lmode from backend apply signature

ElemRestriction - move lmode to constructor over apply

Vector - add additional note on CeedVectorNorm to clarify limitations

Vector - add CeedVectorNorm

Operator - only assemble diag for nonzero QF entries

Restriction - add lmode to ElemRestrictionGetMultiplicity

Operator - fix fallback ceed mem leak

Doc - update doc for make doc errors

Basis - update documentation for CeedMatrixMultiply

Operator - Switch CreateFDMElementInverse to fallback mechanism

Basis - drop BasisGetValue in favor of BasisGetInterp/Grad

Operator - move AssembleDiagonal to fallback

Style - rename files vec->vector for consistency