Add supporting documentation for Macros

[Corallus-Caninus]

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• [x] I have checked the latest main branch to see if this has already been fixed
• [x] I have searched existing issues and pull requests for duplicates

URL to the section(s) of the book with this problem: https://doc.rust-lang.org/book/ch19-06-macros.html Description of the problem: Macros have always been underdocumented in the book which is outstanding otherwise. While Macros may be too much to cover all at once in the book I think further docmentation should be referenced. Suggested fix: Add an appendix with https://github.com/dtolnay/proc-macro-workshop and https://danielkeep.github.io/tlborm/book/README.html referenced for further reading to prevent sending programmers on a search campaign for more information.

[carols10cents]

The Little Book of Rust Macros is already referenced (and there's a newer version, it's no longer maintained by Daniel Keep).

I'll consider whether to add a reference to the proc macro workshop, though. Thanks!

[Corallus-Caninus]

Thanks I had the sneaking suspicion I forgot something. I'll let you close this than.