users-guide: Override mathjax_path
users-guide: Override mathjax_path

The Mathjax CDN no longer exists. Use CDNJS instead.

See #15006.

Test Plan: Build HTML users guide, see whether math is rendered.

Reviewers: alpmestan

Reviewed By: alpmestan

Subscribers: thomie, carter

GHC Trac Issues: #15006

Differential Revision: https://phabricator.haskell.org/D4603

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc