Change static.rust-lang.org to doc.rust-lang.org

The new documentation site has shorter urls, gzip'd content, and index.html
redirecting functionality.
doc: Fix some broken links
doc: Touch up the unsafe guide

* Change ~ references to Box
* Rewrite examples so they can be compiled an run
* Mention libcore
* Update wording about compiler-required functions
auto merge of #14348 : alexcrichton/rust/doc.rust-lang.org, r=huonw