Math characters aren't rendered in sidebars

[choldgraf]

Originally discovered in jupyter-book/jupyter-book#785 if people put math in their section headers or page titles, they won't be rendered in sidebars. E.g.:

====================
About :math:`\alpha`
====================

will be rendered as "About \alpha", not "About 𝛼"

This is true of the sidebar and the table of contents.

Also note this would be fixed by #219

[12rambau]

Tested against v0.8.1, it's working in header sidebar-secodary and page title.

It's still not rendering in header navbar and sidebar-primary.