rxd doc updates including bio faq

[ramcdougal]

Added text, explanation, and structure.

The main change is a significant update to https://nrn.readthedocs.io/en/latest/python/modelspec/programmatic/rxd.html#neuron-rxd

including the addition of a new page for rxdmath.

[codecov-commenter]

Codecov Report

Merging #1680 (0b6680b) into master (e493506) will increase coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1680      +/-   ##
==========================================
+ Coverage   45.30%   45.34%   +0.03%     
==========================================
  Files         551      551              
  Lines      111206   111206              
==========================================
+ Hits        50380    50424      +44     
+ Misses      60826    60782      -44     

Impacted Files	Coverage Δ
share/lib/python/neuron/rxd/export.py	0.00% <0.00%> (ø)
share/lib/python/neuron/rxd/node.py	77.75% <ø> (ø)
share/lib/python/neuron/rxd/species.py	78.78% <0.00%> (ø)
src/parallel/bbs.cpp	75.94% <0.00%> (+1.68%)	⬆️
src/parallel/bbssrvmpi.cpp	48.19% <0.00%> (+6.02%)	⬆️
src/parallel/bbsclimpi.cpp	58.13% <0.00%> (+8.13%)	⬆️
src/nrnmpi/bbsmpipack.cpp	93.33% <0.00%> (+10.76%)	⬆️

📣 Codecov can now indicate which changes are the most critical in Pull Requests. Learn more

[ramcdougal]

The documentation changes are temporarily visible at:
https://nrn.readthedocs.io/en/rxd-docs/

[pramodk]

@adamjhn or @nrnhines: would you be taking a look at this before merge?

[ramcdougal]

This was the one where I had an issue with the push the other day. The addition is contained to the end of the main rxd doc page. I'll resolve that when I go into the office.