This workaround was implemented in cd195279.
The zero-width spaces were inadvertently removed in
248ad188.

Ref. #1707

(cherry picked from commit 6fe43545)

(cherry picked from commit 8a7440c6)

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).

Rather than using a generated file. This will ease the transition to Meson.

It is now supported by the MATLAB editor (as of R2022a).

The old ASCII notation is left in some files that we copy as-is from other
sources (e.g. in the contrib/ and m4/ subdirectories).

The particles submodule is not updated at this point, because it is in an
inconsistent state.

[skip ci]

Does not work with the version of Sphinx used on our Gitlab runners.

This reverts commit 60b31b3b.

The workaround consists in appending the names of those macro-directives with a
regular space followed by a zero-width space (U+200B).

An extra LaTeX declaration was also added in order to make this hack work with
the PDF.

Ref. #1707

- remove trailing whitespace
- replace tabs with spaces as per PEP guidelines

- integrate with autoconf/automake, so that "make html pdf" now build the
  manual
- remove the old texinfo manual
- the PDF manual is now called dynare-manual.pdf (Closes #1543)
- add a Gitlab CI job that updates the online unstable manual under
  www.dynare.org/manual-unstable/