(re-filing of rust-lang/rust#34022)

Cargo allows crate developers to create examples to show off their crate's API (as shown in http://doc.crates.io/guide.html#project-layout). These examples are very useful to see how the individual items in a crate can be used together, in a way that the standard documentation doesn't capture very well.

When viewing a crate's documentation, it would be useful if examples were rendered as standard source files, and there were links to view them on the main library overview page.