👌 IMPROVE: MathJax compatibility with nbsphinx

[chrisjsewell]

If spatialaudio/nbsphinx#577 is accepted, this will make the mathjax processing classes the same for both extensions (both override a global variable)
Also improves the warning, and allows it to be suppressed

[codecov]

Codecov Report

Merging #395 (4796b8f) into master (6eba0a9) will increase coverage by 0.35%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #395      +/-   ##
==========================================
+ Coverage   90.30%   90.65%   +0.35%     
==========================================
  Files          14       14              
  Lines        1846     1851       +5     
==========================================
+ Hits         1667     1678      +11     
+ Misses        179      173       -6     

Flag	Coverage Δ
pytests	90.65% <88.88%> (+0.35%)	⬆️

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Impacted Files	Coverage Δ
myst_parser/main.py	87.50% <ø> (ø)
myst_parser/mathjax.py	95.65% <88.88%> (+15.16%)	⬆️

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