DOC: mention sphinx_immaterial theme

Author: mgeier

doc/usage.ipynb

@@ -436,6 +436,10 @@
     "  [example](https://nbsphinx.readthedocs.io/en/holoviz-theme/),\n",
     "  [usage](https://github.com/spatialaudio/nbsphinx/compare/holoviz-theme^...holoviz-theme)\n",
     "\n",
+    "* [sphinx_immaterial](https://github.com/jbms/sphinx-immaterial):\n",
+    "  [example](https://nbsphinx.readthedocs.io/en/immaterial-theme/),\n",
+    "  [usage](https://github.com/spatialaudio/nbsphinx/compare/immaterial-theme^...immaterial-theme)\n",
+    "\n",
     "* [sphinx_material](https://github.com/bashtage/sphinx-material):\n",
     "  [example](https://nbsphinx.readthedocs.io/en/material-theme/),\n",
     "  [usage](https://github.com/spatialaudio/nbsphinx/compare/material-theme^...material-theme)\n",