🔧 MAINTAIN: Split sphinx extension into separate module

[chrisjsewell]

No description provided.

[codecov]

Codecov Report

Merging #409 (0e2c6d6) into master (3c7fec7) will increase coverage by 0.09%.
The diff coverage is 91.83%.

@@            Coverage Diff             @@
##           master     #409      +/-   ##
==========================================
+ Coverage   80.44%   80.53%   +0.09%     
==========================================
  Files          21       22       +1     
  Lines        2132     2142      +10     
==========================================
+ Hits         1715     1725      +10     
  Misses        417      417              

Flag	Coverage Δ
pytests	80.53% <91.83%> (+0.09%)	⬆️

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Impacted Files	Coverage Δ
myst_nb/docutils_.py	77.94% <ø> (ø)
myst_nb/sphinx_ext.py	91.66% <91.66%> (ø)
myst_nb/__init__.py	71.42% <100.00%> (ø)
myst_nb/sphinx_.py	88.93% <100.00%> (-0.44%)	⬇️

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