update URL for MathJax for doxygen

[ptahmose]

Description

In the doxygen-generated documenation, rendering MathJax did not work any more (because the CDN hosting the package has gone offline). This is supposed to change to an active CDN - c.f. https://www.mathjax.org/cdn-shutting-down/.

Problem can be seen here

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

locally

Checklist:

• I followed the Contributing Guidelines.
• I did a self-review.
• I commented my code, particularly in hard-to-understand areas.
• I updated the documentation.
• I updated the version of libCZI following Versioning of libCZI depending on the type of change
  • Bug fix -> PATCH
  • New feature -> MINOR
  • Breaking change -> MAJOR
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.01%. Comparing base (dedfa6e) to head (3f58a3b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #104   +/-   ##
=======================================
  Coverage   66.01%   66.01%           
=======================================
  Files          85       85           
  Lines       10664    10664           
=======================================
  Hits         7040     7040           
  Misses       3624     3624           

Flag	Coverage Δ
windows-latest	66.01% <ø> (ø)

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