Fix #5497: Do not include MathJax.js and jsmath.js unless it is really needed

[tk0miya]

Feature or Bugfix

• Feature
• Bugfix

Purpose

• refs: Do not include MathJax.js unless it is really needed #5497

[codecov]

Codecov Report

Merging #5536 into 1.8 will decrease coverage by <.01%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##              1.8    #5536      +/-   ##
==========================================
- Coverage   82.04%   82.04%   -0.01%     
==========================================
  Files         306      300       -6     
  Lines       40367    40047     -320     
  Branches     6240     6187      -53     
==========================================
- Hits        33121    32858     -263     
+ Misses       5861     5812      -49     
+ Partials     1385     1377       -8

Impacted Files	Coverage Δ
tests/test_ext_math.py	84.42% <100%> (+1.09%)	⬆️
sphinx/ext/jsmath.py	89.36% <66.66%> (-1.95%)	⬇️
sphinx/ext/mathjax.py	89.06% <66.66%> (-1.42%)	⬇️
sphinx/domains/math.py	74.41% <77.77%> (+0.39%)	⬆️
sphinx/make_mode.py
sphinx/search/__init__.py
sphinx/quickstart.py
sphinx/errors.py
sphinx/__init__.py
sphinx/apidoc.py
... and 1 more

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