Add Rake task for generating docs

[brettchalupa]

I'd like to be able to run something like rake graphql:docs:generate or something similar to run the generator for the docs site.

This would allow people to hook into other Rake tasks to generate the site. Example: Rake::Task["assets:precompile"].enhance(["graphql:docs:generate"])

Tasks:

• [ ] build this out
• [ ] write tests for the rake task
• [ ] document it