text: Title: [DOC] `get_features_targets` should be removed from general_functions.rst
  Body: # Brief Description of Fix
  
  <!-- Please describe the fix in terms of a "before" and "after". In other words, what's not so good about the current docs
  page, and what you would like to see it become.
  
  Example starter wording is provided. -->
  
  When I see docs I found [`get_features_targets` page](https://pyjanitor-devs.github.io/pyjanitor/reference/janitor.functions/janitor.get_features_targets.html) was black.
  
  After #556 merged, there also should remove `get_features_targets` from general_functions.rst.
  But #556 didn't remove it. So this cause that page became blank.
  
  # Relevant Context
  
  <!-- Please put here, in bullet points, links to the relevant docs page. A few starting template points are available
  to get you started. -->
  
  - [`get_features_targets` page is blank](https://pyjanitor-devs.github.io/pyjanitor/reference/janitor.functions/janitor.get_features_targets.html)
  - [general_functions.rst should update](https://github.com/pyjanitor-devs/pyjanitor/blob/dev/docs/reference/general_functions.rst)
  