Feat date-functions article

[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):

• [ ] 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.
• [ ] 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])
• [ ] 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): Added guidance on dates in pyspark and sparklyr

Requirements for review (such as):

• Can you also please review that the new chapter is fine being under spark functions ?

[AntonZogk]

@NathanKelly-ONS i didn't use the converter for this, i created the markdown from scratch

[NathanKelly-ONS]

@NathanKelly-ONS i didn't use the converter for this, i created the markdown from scratch

@AntonZogk I appreciate that it probably took a fair amount of time (and will take even longer to make these changes) but just creating a markdown and uploading it from scratch really isn't the best practice. There's no way to verify that the code actually runs and produces the output that is listed in the output (especially the R code - I know the notebook that this was based on contains the PySpark code) and no trail showing how the page was created. Plus, I imagine there are some users (and also people working on adding additional features to the page) who would want to play around with the code in the notebook which they can't do if we just have the markdown page.

Hope that makes sense - not a priority to work on this if you're busy with other things but when you get a chance it's definitely best practice to create the notebook too.