Document Graphql documentation build rake task

Graphql docs is auto generated using a rake task. We should document how it works so people can find out how to change the template, add resources and update the documentation.

Place where rake tasks are documented: https://docs.gitlab.com/ee/development/rake_tasks.html

The following discussion from gitlab-ce!29998 should be addressed:

• @felipe_artur started a discussion:

  This task should be documented on https://docs.gitlab.com/ee/development/rake_tasks.html