This replaces the old hack which was using a local Linux-specific symlink. Now,
the Dynare default is the same as in Sphinx, i.e. rely on a CDN for delivering
MathJax.

Adapt our CI accordingly to continue using MathJax from the Debian package
there (to avoid privacy issues related to the use of a CDN).

Mostly working. The testsuite remains to be done.

In particular, windows/deps/mkoctfile64 is modified to print its version number
on stdout instead of stderr. This is what the standard mkoctfile does, and it
allows easier capture of the output from Meson.