docs: Python for formula authors

Begin to document the current consensus about correct Python behavior in Homebrew formulas. The libraries advice is a little more experimental than the applications advice but I've started to experiment with it in homebrew-python and nobody's complained yet...!
For brew-packaged dependencies like NumPy, should we vendor them as resources or use a cross-tap dependency? I tested both approaches in https://github.com/Homebrew/homebrew-science/pull/1359 but I would prefer the former. Ideally also want to allow for user-installed modules, either through an option to disable the resource or a `package_installed?` check?