Auto merge of #105763 - Ezrashaw:rustc-book-relative-links, r=jyn514

Use relative links in the unstable and rustc book

Fixes #98227.

As it stands, potential issues could arise from mismatch of stable/ nightly in the books. This PR fixes this issue by using relative links instead of absolute ones. I have replaced every absolute link under `src/doc/rustc` and `src/doc/unstable-book/` and tested each manually.

The link at `src/doc/rustc/src/targets/index.md:7`:
`[here](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_target/spec/struct.Target.html).`
I need help with because my local directory structure and https://doc.rust-lang.org/ seem to be different, where is `nightly-rustc`?

`@jyn514`

Author: bors