docs: do not conflate the generic amulet with the Canton Coin instance

[isegall-da]

In several places in the docs, we currently conflate the Splice Amulet with its specific instantiation as Canton Coin. This should be cleaned up, by making it clear that Canton Coin is just one example of an instance of Amulets.

[waynecollier-da]

This separation is essential, since we can't ever give the impression that Canton Coin is a product of / created by the Splice project.