docs: begin a "low-level & unsafe code" guide.

This aims to cover the basics of writing safe unsafe code. At the moment
it is just designed to be a better place for the `asm!()` docs than the
detailed release notes wiki page, and I took the time to write up some
other things.

More examples are needed, especially of things that can subtly go wrong;
and vast areas of `unsafe`-ty aren't covered, e.g. `static mut`s and
thread-safety in general.

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