website: generate toc data for linking into headers

This adds ids to headers, so you can deep link, which will help when linking people into the documentation.

A note: redcarpet just added an awesome feature that makes the
anchor links human readable. i.e `shell-provisioner` instead of `toc_0`, as it will be as of this pull. See the redcarpet [changelog](https://github.com/vmg/redcarpet/blob/master/CHANGELOG.md#changelog). It's not yet released, but when it is we should upgrade to that, ya!