adding documention and an auth api for boards_controller

adding an auth endpoint to pull big board boards
This is a nitpick, but I think you can keep the original line and just do this:

@boards = Board.where network_id: current_user.network_id
respond_to do |format|
  format.html
  format.json
end

@boards should automatically be available in both cases, and this way you're not duplicating code