fix myst-parser warning blocking docs ci

[awaelchli]

What does this PR do?

The CI on master is failing with the docs make job in CI failing with the warning

WARNING: `mathjax3_config['options']['processHtmlClass']` is being overridden by myst-parser to tex2jax_process|mathjax_process|math. Set `myst_mathjax_classes = None` if this is undesirable.

This PR unblocks master from merging PRs.

Issue opened on myst-parser: executablebooks/MyST-Parser#394

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[codecov]

Codecov Report

Merging #7967 (80ea31b) into master (f15ea60) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #7967   +/-   ##
======================================
- Coverage      91%     91%   -0%     
======================================
  Files         203     203           
  Lines       13129   13129           
======================================
- Hits        11992   11979   -13     
- Misses       1137    1150   +13     

[kaushikb11]

:hand_with_index_and_middle_fingers_crossed: 

[awaelchli]

docker failing with no space left on device, seems unrelated to this PR. everything else looks ok