Local `rustc-docs` could make use of local pages for `std` types

Posting this for @blyxyas and big thanks for having spotted this issue real quick :)

Location (URL)

The same problem persists in many different pages, just to pick an example on my machine:

$RUSTUP_HOME/toolchains/nightly-aarch64-apple-darwin/share/doc/rust/html/rustc-docs/rustc_trait_selection/errors/struct.AmbiguousImpl.html

Summary

The external link for Option<> is https://doc.rust-lang.org/nightly/core/option/enum.Option.html, which is probably desirable for online usage, but considering the fact that this download is intended for offline usage, it'd be better if it can point to the std docs just next to it, i.e. $RUSTUP_HOME/toolchains/nightly-aarch64-apple-darwin/share/doc/rust/html/std/option/enum.Option.html?

A few things to be noted for this issue:

1. This issue should be resolved in a way that it doesn't break the online docs.

2. This does imply an implicit dependency between rustc-docs and rust-docs which rustup is not quite ready to handle, but since rustc-docs is mostly for in-house usage, I believe we can focus on redirecting the links for the time being.

cc @GuillaumeGomez @jieyouxu