Update installation docs with clearer instructions for how to test

[hx2A]

Advise users not to test with the generic Python interpreter. See below.

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I just ran through the install process on my Mac. I run Ventura with Homebrew and all of the pre-reqs are installed. When I run the quick example in a virtual environment and repl, I received the following error from py5:

Unable to obtain source code for setup(). Either make it obtainable or create a settings() function for calls to size(), fullscreen(), etc.
py5 encountered an error in your code:

File "<stdin>", line 2, in setup

RuntimeError: Cannot call the size() method here. Either move it to a settings() function or move it to closer to the start of setup().

I did not see anything specific to this issue in the Mac OSX notes. Is there some other thing I need to do to run this from the python repl?

Originally posted by @drpjm in py5coding/py5generator#337