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

06_text_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/06_text_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 6](https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/master/06_text_00_lecture.ipynb) in the book.\n",
+    "The exercises below assume that you have read [Chapter 6 <img height=\"12\" style=\"display: inline-block\" src=\"static/link_to_nb.png\">](https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/master/06_text_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."
    ]
@@ -34,11 +41,11 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "[Palindromes](https://en.wikipedia.org/wiki/Palindrome) are sequences of characters that read the same backward as forward. Examples are first names like \"Hannah\" or \"Otto,\" words like \"radar\" or \"level,\" or sentences like \"Was it a car or a cat I saw?\"\n",
+    "[Palindromes <img height=\"12\" style=\"display: inline-block\" src=\"static/link_to_wiki.png\">](https://en.wikipedia.org/wiki/Palindrome) are sequences of characters that read the same backward as forward. Examples are first names like \"Hannah\" or \"Otto,\" words like \"radar\" or \"level,\" or sentences like \"Was it a car or a cat I saw?\"\n",
     "\n",
     "In this exercise, you implement various functions that check if the given arguments are palindromes or not. We start with an iterative implementation and end with a recursive one.\n",
     "\n",
-    "Conceptually, the first function, `unpythonic_palindrome()`, is similar to the \"*Is the square of a number in `[7, 11, 8, 5, 3, 12, 2, 6, 9, 10, 1, 4]` greater than `100`?*\" example in [Chapter 4](https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/master/04_iteration_00_lecture.ipynb#Example:-Is-the-square-of-a-number-in-[7,-11,-8,-5,-3,-12,-2,-6,-9,-10,-1,-4]-greater-than-100?): It assumes that the `text` argument is a palindrome (i.e., it initializes `is_palindrom` to `True`) and then checks in a `for`-loop if a pair of corresponding characters, `forward` and `backward`, contradicts that.\n",
+    "Conceptually, the first function, `unpythonic_palindrome()`, is similar to the \"*Is the square of a number in `[7, 11, 8, 5, 3, 12, 2, 6, 9, 10, 1, 4]` greater than `100`?*\" example in [Chapter 4 <img height=\"12\" style=\"display: inline-block\" src=\"static/link_to_nb.png\">](https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/master/04_iteration_00_content.ipynb#Example:-Is-the-square-of-a-number-in-[7,-11,-8,-5,-3,-12,-2,-6,-9,-10,-1,-4]-greater-than-100?): It assumes that the `text` argument is a palindrome (i.e., it initializes `is_palindrom` to `True`) and then checks in a `for`-loop if a pair of corresponding characters, `forward` and `backward`, contradicts that.\n",
     "\n",
     "**Q1**: How many iterations are needed in the `for`-loop? Take into account that `text` may contain an even or odd number of characters! Inside `unpythonic_palindrome()` below, write an expression whose result is assigned to `chars_to_check`!"
    ]
@@ -221,7 +228,7 @@
     "\n",
     "**Q5**: Copy your solutions to the previous questions into `almost_pythonic_palindrome()` below!\n",
     "\n",
-    "**Q6**: The [reversed()](https://docs.python.org/3/library/functions.html#reversed) and [zip()](https://docs.python.org/3/library/functions.html#zip) built-ins allow us to loop over the same `text` argument *in parallel* in both forward *and* backward order. Finish the `for` statement's header line to do just that!\n",
+    "**Q6**: The [reversed() <img height=\"12\" style=\"display: inline-block\" src=\"static/link_to_py.png\">](https://docs.python.org/3/library/functions.html#reversed) and [zip() <img height=\"12\" style=\"display: inline-block\" src=\"static/link_to_py.png\">](https://docs.python.org/3/library/functions.html#zip) built-ins allow us to loop over the same `text` argument *in parallel* in both forward *and* backward order. Finish the `for` statement's header line to do just that!\n",
     "\n",
     "Hint: You may need to slice the `text` argument."
    ]
@@ -454,7 +461,7 @@
    "source": [
     "**Q11**: Copy your code from `pythonic_palindrome()` above into `palindrome_ducks()` below and make the latter conform to *duck typing*!\n",
     "\n",
-    "Hints: You may want to use the [str()](https://docs.python.org/3/library/functions.html#func-str) built-in. You only need to add *one* short line of code."
+    "Hints: You may want to use the [str() <img height=\"12\" style=\"display: inline-block\" src=\"static/link_to_py.png\">](https://docs.python.org/3/library/functions.html#func-str) built-in. You only need to add *one* short line of code."
    ]
   },
   {
@@ -576,7 +583,7 @@
    "source": [
     "**Q13**: Implement the final iterative version `is_a_palindrome()` below. Copy your solution from `palindrome_ducks()` above and add code that removes the \"special\" characters (and symbols) from the longer example palindromes above so that they are effectively ignored! Note that this processing should only be done if the `ignore_symbols` argument is set to `True`.\n",
     "\n",
-    "Hints: Use the [replace()](https://docs.python.org/3/library/stdtypes.html#str.replace) method on the `str` type to achieve that. You may want to do so within another `for`-loop."
+    "Hints: Use the [replace() <img height=\"12\" style=\"display: inline-block\" src=\"static/link_to_py.png\">](https://docs.python.org/3/library/stdtypes.html#str.replace) method on the `str` type to achieve that. You may want to do so within another `for`-loop."
    ]
   },
   {