The global approach in 395 isn't really sustainable: it requires all-ways
cooperation between all projects that want to customize MathJax.  Additionally,
when processing a MyST document without Sphinx, the MathJax configuration
changes are not performed (part of executablebooks#347).  And, of course, this approach of
overriding the MathJax object causes issues down the line for projects that need
to customize MathJax (the setting in Sphinx isn't sufficient, see sphinx-doc/sphinx#9450)

The following two approaches would not cause these issues:

1. Add a custom script instead of touching the mathjax3_config variable;
   something like this, essentially:

   ```js
   app.add_js_file(None, priority=0, body="""
      var MathJax = window.MathJax || MathJax;
      MathJax.options = MathJax.options || {};
      MathJax.options.processHtmlClass = (MathJax.options.processHtmlClass || "")
      + "|math";
   """)
   ```

- Don't touch MathJax_config at all; instead, add an explicit `mathjax_process`
  class on all math nodes, either by changing `docutils_renderer` (this PR) or by
  adding a Docutils transform to processes all math nodes:

  ```python
  class ActivateMathJaxTransform(Transform):
      default_priority = 800

      @staticmethod
      def is_math(node):
          return isinstance(node, (math, math_block))

      def apply(self, **kwargs):
          for node in self.document.traverse(self.is_math):
              node.attributes.setdefault("classes", []).append("mathjax_process")
  ```

This PR isn't ready for merging; it's just to start a discussion.