text: Title: [DOCS] Clarify difference in function between data/external and data/raw
  Body: Based on the `<-` descriptions in the 'Directory structure' section of the documentation, there doesn't seem to be clear-cut criterion for choosing between `data/external` and `data/raw` in those cases where the original data dump *originates* from a third-party source, i.e., fulfills the conditions for inclusion in either directory.
  
  What sort of criteria do you apply in such ambiguous cases?
  
  On a related note, where does `data/external` fit in your 'mental model' of the preprocessing pipeline? (Pick one)
  
  - A
  
  ```
  raw ---> interim ---> processed ---+
                                     |
                                     +---> [ analysis ]
                                     |
                        external ----+
  ```
  
  - B
  
  ```
  raw -------+
             |
             +--> interim ---> processed ---> [ analysis ]
             |
  external --+
  ```