Allow overriding docs.rs URLs

[gautamg795]

This is a follow-up request to #12867.
Our organization has documentation hosted internally for our Rust workspace (built via cargo doc --workspace --document-private-items), so most users don't have docs built locally.
Right now, the autogenerated docs.rs URL is invalid for any of our internal crates, so it'd be very helpful to override the "base URL" or provide a format string to get to the docs for a crate.

[Veykril]

Does using #![doc(html_root_url = "...")] not work for you?

[gautamg795]

We'd have to insert that on every crate, I think.
But also I'm not sure it works at all for us from reading the docs -- it seems that it affects how docs are generated; our docs are already generated just fine in CI and hosted on GH Pages. We just want rust-analyzer / VSCode to open a different base URL.

Maybe your suggestion is to have a html_root_url set conditionally so it's not set in CI, and perhaps that would work? But having to insert the attribute on every crate is still not ideal.

That said, I'm happy to take a look at what the implementation for this would look like!

[davidbarsky]

I think I have a branch that allows overriding docs.rs somewhere, lemme see if I can dig it up.