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

00_intro_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/00_intro_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 0](https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/master/00_introduction_00_lecture.ipynb) in the book."
+    "The exercises below assume that you have read [Chapter 0 <img height=\"12\" style=\"display: inline-block\" src=\"static/link_to_nb.png\">](https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/master/00_introduction_00_content.ipynb) in the book."
    ]
   },
   {
@@ -46,7 +53,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    " "
+    " < your answer >"
    ]
   },
   {
@@ -60,7 +67,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    " "
+    " < your answer >"
    ]
   },
   {
@@ -74,7 +81,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    " "
+    " < your answer >"
    ]
   }
  ],