Add "View Source" link

[sindresorhus]

It would be nice if each type had a "View Source" link (could be on hover or in a … menu) that linked to its source (with the correct SHA reference) on GitHub. That way, users could quickly jump into the source

As with #117, this would need some kind of githubRepoURL config.

For example, Rust docs have this: https://docs.rs/gifski/0.10.4/gifski/ (Click the [src] link)