Document ERC721ReceiverComponent

[ericnordelo]

Fixes #913

This PR adds the missing documentation for the ERC721ReceiverComponent, and also
fixes the name of the mixin implementation.

PR Checklist

• Tests
• Documentation
• Added entry to CHANGELOG.md
• Tried the feature on a public network

[andrew-fleming]

Looking good, sir! I left a few suggestions—the biggest one being that we combine the snake and camel embeddable blocks into a single embeddable block. I think we can update Creating a token receiver contract to use ERC721ReceiverMixinImpl as well to clean up the snippet. WDYT?

[ericnordelo]

I think we can update Creating a token receiver contract to use ERC721ReceiverMixinImpl as well to clean up the snippet. WDYT?

The only issue I see with the approach is inconsistency. For example, the ERC721 in the same page is not using mixins, and I think we should update that as well. Let's let it for a different issue if you agree.

#948

[andrew-fleming]

I left a couple more small suggestions and they need to be applied in #941 as well. This looks good to me otherwise