Modernize lists

Add improvements from a few years of in-person courses (wow, it's been a few years already!)
The main change is that the nested lists section is moved to its own lesson. It's good to have a little pause before going through it.

The commits represent historical evolution, don't review them individually.

#9

Author: encukou

courses/pyladies.yml

@@ -100,6 +100,7 @@ plan:
   materials:
   - lesson: beginners/list
   - lesson: beginners/tuple
+  - lesson: beginners/nested-list
   - title: Tahák na seznamy
     url: https://pyvec.github.io/cheatsheets/lists/lists-cs.pdf
     type: cheatsheet