Add a notebook about differential calculus (derivatives and gradients)

Author: ageron

index.ipynb

@@ -38,6 +38,7 @@
     "\n",
     "## Math Tutorials\n",
     "* [Linear Algebra](math_linear_algebra.ipynb)\n",
+    "* [Differential Calculus](math_differential_calculus.ipynb)\n",
     "\n",
     "## Extra Material\n",
     "Work in progress\n",