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!

website/config.rb

@@ -45,7 +45,9 @@ set :images_dir, 'images'
 
 # Use the RedCarpet Markdown engine
 set :markdown_engine, :redcarpet
-set :markdown, :fenced_code_blocks => true
+set :markdown,
+    :fenced_code_blocks => true,
+    :with_toc_data => true
 
 # Build-specific configuration
 configure :build do