Auto merge of #4462 - behnam:book-fixes, r=alexcrichton

[doc/book] Drop reference/policies.md and small fixes

We don't need to put `policies.md` in the book. See
<https://github.com/rust-lang/crates.io/issues/1030> for more.

Renaming `introduction.md` to `index.md` prevents creating two html
files (and URL) for one source.

The rest are small styling issues.

Tracker: <https://github.com/rust-lang/cargo/issues/4040>