This adds documentation for User Roles. While this is a Power Pack feature,

moving forward we'll be consolidating Power Pack documentation into the

Review Board docs base. This follows our plans of unifying things under

just one "product" and will make things easier to find for users.
Along with the user roles documentation, this adds a "Useful Data and

Resources" section to the review request approval hook docs that lists

some attributes and resources that are useful for approval hooks. I added

this because I thought it'd be a good place to plug user roles.
We also now allow references to the Power Pack docs.

Built the docs and viewed them.
Was able to click on the UserRole.objects.for_user() reference

  and get redirected to the Power Pack coderef entry for it.