Make links look nice with images

- rename all *_00_lecture.ipynb files into *_00_content.ipynb
- add links to YouTube videos at the end of the chapters
- add links to mybinder for all chapters and exercises
- remove the old chapter 8 on mappings & sets
- update the README.md and make it look nicer

02_functions_02_exercises.ipynb

@@ -1,5 +1,12 @@
 {
  "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "**Important**: Click on \"*Kernel*\" > \"*Restart Kernel and Run All*\" *after* finishing the exercises in [JupyterLab <img height=\"12\" style=\"display: inline-block\" src=\"static/link_to_jp.png\">](https://jupyterlab.readthedocs.io/en/stable/) (e.g., in the cloud on [MyBinder <img height=\"12\" style=\"display: inline-block\" src=\"static/link_to_mb.png\">](https://mybinder.org/v2/gh/webartifex/intro-to-python/master?urlpath=lab/tree/02_functions_02_exercises.ipynb)) to ensure that your solution runs top to bottom *without* any errors"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},
@@ -18,7 +25,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "The exercises below assume that you have read [Chapter 2](https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/master/02_functions_00_lecture.ipynb) in the book.\n",
+    "The exercises below assume that you have read [Chapter 2 <img height=\"12\" style=\"display: inline-block\" src=\"static/link_to_nb.png\">](https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/master/02_functions_00_content.ipynb) in the book.\n",
     "\n",
     "The `...`'s in the code cells indicate where you need to fill in code snippets. The number of `...`'s within a code cell give you a rough idea of how many lines of code are needed to solve the task. You should not need to create any additional code cells for your final solution. However, you may want to use temporary code cells to try out some ideas."
    ]
@@ -34,7 +41,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "**Q1**: The [volume of a sphere](https://en.wikipedia.org/wiki/Sphere) is defined as $\\frac{4}{3} * \\pi * r^3$. Calculate this value for $r=10.0$ and round it to 10 digits after the comma. Use the [standard library](https://docs.python.org/3/library/index.html) to obtain a good approximation of $\\pi$."
+    "**Q1**: The [volume of a sphere <img height=\"12\" style=\"display: inline-block\" src=\"static/link_to_wiki.png\">](https://en.wikipedia.org/wiki/Sphere) is defined as $\\frac{4}{3} * \\pi * r^3$. Calculate this value for $r=10.0$ and round it to 10 digits after the comma. Use the [standard library <img height=\"12\" style=\"display: inline-block\" src=\"static/link_to_py.png\">](https://docs.python.org/3/library/index.html) to obtain a good approximation of $\\pi$."
    ]
   },
   {
@@ -170,9 +177,9 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "**Q5**: Using the [range()](https://docs.python.org/3/library/functions.html#func-range) built-in, write a `for`-loop and calculate the volume of a sphere with `radius = 42.0` for all `digits` from `1` through `20`. Print out each volume on a separate line.\n",
+    "**Q5**: Using the [range() <img height=\"12\" style=\"display: inline-block\" src=\"static/link_to_py.png\">](https://docs.python.org/3/library/functions.html#func-range) built-in, write a `for`-loop and calculate the volume of a sphere with `radius = 42.0` for all `digits` from `1` through `20`. Print out each volume on a separate line.\n",
     "\n",
-    "Note: This is the first task where you need to use the built-in [print()](https://docs.python.org/3/library/functions.html#print) function."
+    "Note: This is the first task where you need to use the built-in [print() <img height=\"12\" style=\"display: inline-block\" src=\"static/link_to_py.png\">](https://docs.python.org/3/library/functions.html#print) function."
    ]
   },
   {