Intellij-rust: Quick documentaion should contain link to the on-line docs

[matklad]

PyCharm has a useful feature where it adds an "View external documentaion" link to the quick-doc popup (Ctrl+Q):

I think we should do the same for Rust, using docs.rs and https://doc.rust-lang.org/std/.

The entry point for the relevant feature is org.rust.ide.docs.RsDocumentationProvider.

[matklad]

Wow, turns out we have a dedicated shortcut for viewing external docs: https://www.jetbrains.com/help/idea/viewing-external-documentation.html, which is used by PyCharm, for example.

We need to do this as well, though it's E-less-easy, because I don't know off hand which API we should use.

[matklad]

Looks like we need to implement ExternalDocumentationHandler.

[Kobzol]

This seems to be resolved (https://github.com/intellij-rust/intellij-rust/pull/2555).