[mlir] Fix MathJax rendering in Affine doc

MathJax is not properly imported in Affine doc. It causes the invalid rendering
of math formulas in the Affine doc page.

https://mlir.llvm.org/docs/Dialects/Affine/#affine-expressions

Importing MathJax code from CDN resolved the rendering issue as follows.

{F14942131}

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D94004