Move FFI documentation from the Book to the Nomicon

[QuietMisdreavus]

After some discussions in the docs team meetings about #31227, it became clear that FFI documentation doesn't really have a good place any more. The second edition of TRPL decided not to include a follow-up to the FFI discussion that the first edition had. Therefore, once that gets published and more or less supplants the first edition, we risk losing that documentation entirely. Moving it to the Nomicon (in lieu of its own standalone book or some other location) does make some sense, as it requires an understanding of unsafe code.

From there it can be expanded with more up-to-date information, possibly also including extra sources like the Rust FFI Omnibus or similar projects.

[QuietMisdreavus]

Getting this working basically involves copying the FFI chapter out of rust-lang/book and into rust-lang-nursery/nomicon. In the docs team discussion, @steveklabnik said it would be best to leave the 1st edition book as-is, since it's intended to be fairly static now that the 2nd edition is nearly finished, so you wouldn't even need to take it out from the book.