Add training instructions for Cell Painting CNN-1 / Issue on page /docs/07-train.html

[jccaicedo]

Include general instructions, a pointer to the configuration file and a pointer to the dataset.

[jccaicedo]

This can be a subsection in Chapter 7, with all the info and pointers to train your own Cell Painting CNN-1.
Critically, we need pointers to download the dataset @Arkkienkeli