Change headings for better layout of index in documentation

Author: Birgit Schachler

example/basic_example.ipynb

@@ -4,15 +4,12 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
+    "# Basic usage\n",
+    "\n",
     "This example shows you the basic usage of the windpowerlib. \n",
-    "There are mainly three steps. First you have to import your weather data, then you need to specify your wind turbine, and in the last step call the windpowerlib functions to calculate the feedin timeseries."
-   ]
-  },
-  {
-   "cell_type": "markdown",
-   "metadata": {},
-   "source": [
-    "### Import necessary packages and modules"
+    "There are mainly three steps. First you have to import your weather data, then you need to specify your wind turbine, and in the last step call the windpowerlib functions to calculate the feedin timeseries.\n",
+    "\n",
+    "But first of all you need to import the packages needed for the different steps."
    ]
   },
   {