Add a docs section on .values

[TomAugspurger]

• Explain that Series / Index are containers for ndarrays and ExtensionArrays
• Recommend np.asarray for ensuring you get an ndarray
• Explain that DataFrame.values may not be what you want

This may go in dsintro, maybe a new page if we think it's too advanced.

[TomAugspurger]

I think this was handled by
#23623