Creating a flag with age-diff example

[alexsnowdon]

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.
• [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):

• to add a new page to the book to capture information from out GitLab: https://gitlab-app-l-01/DAP_CATS/troubleshooting/python-troubleshooting/-/blob/master/age_diff_flag.ipynb?ref_type=heads

Things to note about this request:

• the 1 update to the ons-spark/ancillary-topics/creating-a-SQL-view-in-HDFS.ipynb file has already been pushed to main from another branch (called png). There are no conflicts with the main branch so this shouldn't cause any issues upon merge.
• the first commit message for the changes linked to the age-diff work is called: new notebook for age-diff-flag info [e905411]

Requirements for review (such as):

• run the python code in the flags.ipynb notebook
• check that all links work
• check that the toc.yml has been updated correctly to include the flags page in the spark analysis section
• perform build of book to check formatting and links again
  • once reviewed, and happy with changes, please complete merge request and delete this branch.

[AntonZogk]

@NathanKelly-ONS , updated the context per your comments and also added the equivalent R code, as it stands it needs the converter doesn't it ? If so we should move it to raw-notebooks and run the converter.