👌 IMPROVE: Do not let sphinx check the config type

[chrisjsewell]

This is already validated by myst-parser

closes #558

[codecov]

Codecov Report

Merging #559 (8909de2) into master (8854d84) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #559   +/-   ##
=======================================
  Coverage   89.64%   89.64%           
=======================================
  Files          17       17           
  Lines        2154     2154           
=======================================
  Hits         1931     1931           
  Misses        223      223           

Flag	Coverage Δ
pytests	89.64% <100.00%> (ø)

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Impacted Files	Coverage Δ
myst_parser/__init__.py	88.57% <100.00%> (ø)

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