GH-115986 Reorder pprint docs and amend some references (#116019)

Introduce a new subsubsection, 'Functions', for module level functions,                                   
and place it before the PrettyPrinter class reference.                                                    
                                                                                                          
Also:
- Fix pprint.pprint() references so they properly link to the module                                   
  level function.
- Add links to sys.stdout.