Auto merge of #2688 - carols10cents:docs-refresh, r=alexcrichton

Auto merge of #2688 - carols10cents:docs-refresh, r=alexcrichton

Docs refresh

Hi! Sooooo I've been thinking about the cargo docs lately and today I decided to try some of the things I've been thinking about.

There are some smaller changes in here, typos and clarifications and stuff, that I'm totally willing to break out into a separate PR(s) if yinz want.

My goals for the larger changes were:

- **Make one obvious place to go for info on every which way to specify a dependency.** I've found myself clicking around the different doc pages a lot to find the semver syntax, the different keys that can go with a git location, the `path` override syntax, etc. So I made one top-level "Specifying Dependencies" page and moved relevant info from the guide, the manifest page, and the crates.io page.
- **Make The Guide a bit more focused.** There were a few topics in there that didn't really fit in the story arc of trying to get someone up and running with cargo-- it veered off into some more advanced topics that I've moved to places I think they fit better.
- **Make the crates.io page only about publishing, not using and publishing.** The info about adding dependencies got folded into either the guide or the new specifying dependencies page, so this should close https://github.com/rust-lang/cargo/issues/1035.

I'm also thinking about having someone new to rust/cargo try following the guide, but wanted to get a first pass from experienced people :)