add intro item
"1. [SkyModel](#SkyModel)\n",
"1. [Modifying model parameters](#Modifying-model-parameters)\n",
"1. [Model Lists and Serialisation](#Model-Lists-and-Serialisation)\n",
+ "1. [Models with shared parameter](#Models-with-shared-parameter)\n",
"1. [Implementing as Custom Model](#Implementing-a-Custom-Model)\n",
"1. [Energy dependent models](#Models-with-energy-dependent-morphology)\n",
"\n",
