[documentation] Saved network format

[harold]

Make a document explaining the output of execute/train / compute-binding/save-to-network.

Explain:
Why do these functions return both a network and an optimizer?

What is the structure of the returned network (it's a map with ... keys, etc...)

Maybe: Which graph functions might be useful to call on a saved network? (e.g., for stripping layers, fine-tuning, etc...)