Write the tutorials

[nilamo]

The Contributing doc mentions a Tutorials repo (https://github.com/ppb/pursuedpybear/blob/canon/CONTRIBUTING.md#i-want-to-teach), and the docs have a Tutorial page (https://ppb.readthedocs.io/en/stable/tutorials/index.html), but both are non-existent or empty. Have the tutorials been removed? Are the links broken?

Currently, all of the docs seem to be a Getting Started guide, and an API reference.