Support for docs.rs links

[jyn514]

I'd like to turn this link:

/// [`Span`]: https://docs.rs/tracing-core/latest/tracing_core/span/index.html

into this one:

/// [`Span`]: tracing_code::span

I think this should be fairly easy to add: if the URL starts with https://docs.rs, strip the host and first 2 path segments, then treat it like a normal link.

[poliorcetics]

Maybe a new option for this ? I can't think of some situation where one might want to link to docs.rs without having the linked crate as a dependency, but there might be one.

Maybe have the option to turn it off and have it on by default is better.

[poliorcetics]

This should work now, and with lots of tests. :)