Add docs about new mathmpl options.

Author: QuLogic

doc/users/next_whats_new/mathmpl_hidpi.rst

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+More configuration of ``mathmpl:`` sphinx extension
+---------------------------------------------------
+
+The `matplotlib.sphinxext.mathmpl` sphinx extension supports two new
+configuration options that may be specified in your ``conf.py``:
+
+- ``mathmpl_fontsize`` (float), which sets the font size of the math text in
+  points;
+- ``mathmpl_srcset`` (list of str), which provides a list of sizes to support
+  `responsive resolution images
+  <https://developer.mozilla.org/en-US/docs/Learn/HTML/Multimedia_and_embedding/Responsive_images>`__
+  The list should contain additional x-descriptors (``'1.5x'``, ``'2x'``, etc.)
+  to generate (1x is the default and always included.)