Add hidden link target to docs

The documentation update explaining the new handling of hash valued
attributes included changing the heading for that section, which means
the generated `id` value will also change and any existing links using
the old value will stop wotking.

Add a new (hidden) link target at the same point with the old `id` so
that those links will continue to work.
Delete callbacks.rb

define_callback is from Sass and is not used in Haml.