docs: a bit more intuitive docs links

[sanmai-NL]

Use a docs.rs badge where appropriate. This is all subjective of course, I feel this is a bit nicer a way to present these links.

• The commit messages match the guidelines in https://github.com/hyperium/hyper/blob/master/CONTRIBUTING.md#git-commit-guidelines

[seanmonstar]

Sure, seems good, thanks!