Add a link to the GitHub web editor for the current documentation

[jyn514]

@SOF3 brought this up in #contribute: currently, making a PR to improve the documentation for a crate is somewhat tedious: find the link to the GitHub project, find where the documentation lives in the code (which doesn't always correspond to the module structure, especially with macros), fork the project, and only then make the edits. It would be nice to have a direct link to the GitHub web editor, which lets you suggest fixes immediately without all the setup.

Doing this per-page isn't so hard, but per-item would require rustdoc support. It would also be nice to have rustdoc support in general, since that would let it be used for the standard library as well.

[Kixiron]

This sounds difficult since releases aren't tied to a particular git hash and the code in a repo isn't always in line with the posted documentation. Other than that, love the idea and it'd be really fantastic

[jyn514]

A simpler idea that wouldn't need changes to the UI is to change the [src] links to point directly to github, and let github take care of editing.

[SOF3]

For [src] we can be confident that the link is valid. Changing a confidently valid link to a probably invalid link sounds like a regression.