Update references that may be obsolete
Update references that may be obsolete

Sorry, I don't have bandwidth to make this perfect immediately.
If you want to resurrect some contents, please file a pull request.

However, removing Haml helpers was intentional in Haml 6. Mentions of
helpers are not supposed to be resurrected.

haml/haml,haml/haml,haml/haml,haml/haml