Add support for "{target}" to the `--extern-html-root-url` cargo passes to rustdoc (through `doc.extern-map.registries`)

[jyn514]

Describe the problem you are trying to solve
docs.rs would like to link to builds for specific targets, since not all targets have the same documentation: rust-lang/docs.rs#1397 (comment). However, this requires more flexibility in the URL than cargo currently allows. It would be great if -Zrustdoc-map added support for this so we don't have to go back to adding --extern-html-root-url by hand (which was quite buggy).

Describe the solution you'd like
Add support for {target} to doc.extern-map.registries. It should have the following behavior:

1. When using cargo doc --target x, replace {target} with x in the remapping.
2. When using cargo doc alone, without --target, remove {target} altogether.

I don't have strong opinions on 2 (docs.rs will work even if it's replaced with the host target) but it seems like a reasonable behavior.

Notes

I am interested in implementing this myself.

[jyn514]

Err actually I just realized docs.rs can do this without changes to cargo, so it's probably not necessary, we can just pass --config=doc.extern-map.registries.crates-io="https://docs.rs/{pkg_name}/{version}/hardcoded_target".

[jyn514]

Err @Nemo157 mentions this could still be useful for people other than docs.rs (e.g. building a crate on a target other than the package.metadata.docs.rs.default-target should probably link to that specific target). So it would be nice for this to be part of the default map for docs.rs.