website: generate toc data for linking into headers

A note: redcarpet just added an awesome feature that makes the
anchor links human readable. i.e `shell-provisioner` instead of `toc_0`.

see: https://github.com/vmg/redcarpet/blob/master/CHANGELOG.md#changelog

It's not yet released, but when it is we shpould upgrade to that, ya!
Merge pull request #305 from mitchellh/website-deep-links

website: generate toc data for linking into headers