WIP: Refactor keys API documentation

There are two resources for keys:

• https://gitlab.com/gitlab-org/gitlab-ce/blob/ebaedca907ca4387427ac98c8b92baba859f96d4/lib/api/users.rb#L323-371
• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/api/keys.rb

TODO:

• Add a note in keys.md where to find the user's keys calls and vice versa