Recipe documentation

[joecummings]

1. Add recipe documentation to torchtune
2. Remove docstrings from recipe classes
3. Profit

cc @SalmanMohammadi

[felipemello1]

I am not sure if i like it. If the documentation stays close to the code, its easier to keep it updated and understand whats going on. Moving elsewhere means that users may not know it exists and they may get out of sync. I am happy to be convinced otherwise, but usually, i prefer to generate documentation from code.