[Style guide] Update entry on 'Abbreviations' to include guidance on days of the week

[alexhallspringer]

What

Update entry on 'Abbreviations' to include guidance on days of the week.

Why

For completeness and consistency.

Done when

• [x] Review existing usage.
• [x] Review external guidance.
• [x] Draft guidance.
• [x] Entry is added to the style guide.

Anything else

Go to this entry in the proposed style guide entries Google sheet

Proposed entry content

Do not abbreviate days of the week or months of the year. Write them out in full.