Use cargo's rustdoc-map instead to link to dependencies #1177
Description
Currently we're generating --extern-html-root-url flags to pass to rustdoc ourselves:
docs.rs/src/docbuilder/rustwide_builder.rs
Lines 521 to 529 in b527df8
This has issues (#1166, #1165).
Cargo has an unstable rustdoc-map feature rust-lang/cargo#8296 intended to allow users to do a similar thing easily via a simple url template. We can likely use this instead of doing it manually.