Add sphinx.ext.mathjax to fix formula rendering

[shralex]

Adds sphinx.ext.mathjax to the Sphinx extensions in conf.py to enable formula rendering in the documentation. This fixes issues where formulas parsed by MyST's dollarmath were not displayed correctly in the HTML output on ReadTheDocs.

FIXES: b/501246406

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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