There's a lot of great ideas here! I'd like to bring up 1 specific example:
From the serde page on crates.io you see two links, one for Documentation and another for Repository
The landing page for the serde rustdocs is a bit lackluster, with only a brief introductory paragraph, and nothing at all about how to use the library.
But if you click on the Repository link, you are brought to the github page with a most wonderful README, full of exactly the information a new user would want to see.
How can we consolidate this, and remove the need for a user two go to two different places for docs (one of which isn't even an obvious place for rich documentation, even though it is). Maybe by allowing rustdoc to pull in a README file?