text: Title: Display warning when no recommendations are generated
  Body: When no recommendations are generated (e.g., when [dataframe is small but not preaggregated](https://github.com/lux-org/lux/blob/master/lux/core/frame.py#L153), possibly other cases), we should display a warning that explains why the Lux view is not showing up. 
  
  Add an advanced ReadTheDoc page explaining default recommendation logic, including when recommendations are *not* displayed.
  
  _Originally posted by @akanz1 in https://github.com/lux-org/lux/issues/110#issuecomment-706659586_