Add links to original Halogen examples

[milesfrain]

The Halogen-hooks recipes that are based on the Halogen-classic examples should include a link to these in their readmes.

cookbook hook recipe --> classic repo example

Perhaps the Halogen-classic examples should be duplicated in the cookbook too, in which case we could link as:

cookbook hook recipe <--> cookbook classic recipe --> classic repo example

Originally proposed in https://github.com/JordanMartinez/purescript-cookbook/issues/181#issuecomment-659844137

[JordanMartinez]

Let's not duplicate the "halogen-classic" examples here as a way to reduce maintenance on our part.