Groups not loops

[jkhall06]

Summary of changes

• Added the groups-not-loops.ipynb notebook to raw-notebooks/groups-not-loops folder.
• Now contains r code in the notebook. I am unable to run the notebook converter due to access permissions on my laptop, please could reviewer run the converter and build the book to ensure the code displays correctly. The output_path for converted md file should be: ons-spark/spark-concepts/
• Changed the ToC to reflect new section. It comes before the shuffling section as I thought it fitted best there before going into more detail on how wide operations work in Spark (shuffling, partitioning).

[AntonZogk]

Merge request template: please remove the appropriate parts of this template.

Pre-merge request checklist (to be completed by the one making the request):

• [x] I have performed a full review of this code myself.
  • For Python code in PySpark specific sections, all code should have been run in Jupyter notebooks.
  • For code in sections of the book containing both Python and R code, the page of the book should be constructed as described in the contributing guide and converted to a markdown file.
• [x] I have formatted the outputs of code blocks correctly (to match other outputs in the book and in line with the style guide [coming soon])
• [x] I have built the book as outlined in the contributing guide and confirmed that any additional/modified content is displaying as expected.

Details of this request (such as): N/A

Things to note about this request (such as): N/A

Requirements for review (such as):

• Converter successfully run
• Built as expected