Improved documentation of training steps, with examples

[bnb32]

sup3r version checks

• I have checked that the issue still exists on the latest versions of the docs on main here

Location of the documentation

https://nrel.github.io/sup3r/examples/examples.html

Documentation problem

Setting up a training loop is complicated and deserves at least one example outside of the tests.

Suggested fix for documentation

Add a script or a notebook with an end to end training setup / loop in an examples/training directory.