rustdoc should add repository links automatically

[lzybkr]

It's handy to jump from docs to a repo, e.g. to explore/open issues, find examples, etc.

As an example of what this might look like, see anyhow.

Can the repository field be extracted from Cargo.toml easily enough so this wouldn't require any user changes to enable?

[GuillaumeGomez]

It's not up to rustdoc but to cargo. rustdoc doesn't use the Cargo.toml file.

[lzybkr]

Would it make sense for rustdoc to add a command line parameter --repository so cargo can pass the link?

[GuillaumeGomez]

I guess it'd be acceptable yes.

[lolbinarycat]

triage: docs.rs has this information in its toolbar, is that good enough?