Add nbsphinx_assume_equations to documentation (#604)

Author: diego-plan9

doc/usage.ipynb

@@ -248,6 +248,15 @@
     "See [Ignoring Errors](allow-errors.ipynb)."
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "#### `nbsphinx_assume_equations`\n",
+    "\n",
+    "If `False`, do not force loading MathJax on HTML pages generated from notebooks."
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},