Use relative links in the unstable book

[jyn514]

Using absolute links to https://doc.rust-lang.org/stable/rustc means that the channel used in the link may not match the channel where the docs are hosted. This is especially bad for the unstable docs, which can change a lot between stable and nightly. We should switch these to using relative links instead.

Originally posted by @ChrisDenton in #98226 (comment)

[Ezrashaw]

I assume this still can be worked on.

@rustbot claim