Adding a note about the related `get_pixels()` methods on the `copy()` documentation pages

[villares]

This is a PR attempt based on our discussion here: https://github.com/py5coding/py5generator/discussions/624 I hope it helps someone reading the docs... I'm not sure anymore...

I'm unsure about some stylistic details (such as adding a dot to the references of method names...) and about the magical links that pybook likes (no /reference/ at the path to the linked pages, or should I add them?).

This is not very urgent stuff... :)