Optimising joins parquet

[NathanKelly-ONS]

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): Modified the optimising joins chapter to read in a parquet file rather than a CSV file. Also updated the Spark UI screenshots.

Requirements for review (such as):

• Build book, check code and images display properly
• Compare images against previous chapter to ensure they're displaying the same content (with the exception of CSVs being replaced with parquets)

[NathanKelly-ONS]

@alexsnowdon I made your suggested changes, do you mind giving them a review and merging them in when you get a chance :)

[alexsnowdon]

@NathanKelly-ONS I can see you have made the suggested changes, unfortunately when I build the book figure 18 and figure 21 are now broken. For figure 18 it seems that in the .md file and in the .ipynb the figure is referenced differently. In the .md a 'soft_merge_join_ui_new.png' is referenced, whereas in the images folder and in the .ipynb this is just 'sort_merge_join_ui.png'! For figure 21, I cannot see what could be causing this as in all files it is referenced as when_ui.png.

[NathanKelly-ONS]

@alexsnowdon finally managed to get it to work (I think!). It's working on my laptop and I've built and checked that all images are displaying, and they seem to be. Hopefully it's all good on your end too :)