19792: Revamp the Python SDK cookbook

19792: Revamp the Python SDK cookbook

* Organize the recipes into related sections based on what they work
  on.

* Add a preface to each recipe to explain when you might use it, give
  API background, etc.

* Write the recipes in a style that emphasizes readability with long
  variable names, lots of line breaks, and comments highlighting
  key mechanics and variations.

* Modernize SDK usage in the recipes.

* Add links to supporting documentation where available.

As much as practical, I have maintained existing anchor names, even when
I updated header names for more consistent style.

Arvados-DCO-1.1-Signed-off-by: Brett Smith <brett.smith@curii.com>