Auto merge of #38770 - steveklabnik:doc-custom-derive, r=alexcrichton

Document custom derive.

These are some bare-bones documentation for custom derive, needed
to stabilize "macros 1.1",
https://github.com/rust-lang/rust/issues/35900

The book chapter is based off of a blog post by @cbreeden,
https://cbreeden.github.io/Macros11/

Normally, we have a policy of not mentioning external crates in
documentation. However, given that syn/quote are basically neccesary
for properly using macros 1.1, I feel that not including them here
would make the documentation very bad. So the rules should be bent
in this instance.

So far, this PR includes only docs; @alexcrichton said in https://github.com/rust-lang/rust/issues/35900 that he'd be okay with landing them before stabilization; I don't mind either way.

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