[Style guide] Add 'ampersands' as an entry in the A to Z

[alexhallspringer]

What

Add guidance on ampersand to the style guide

Why

For completeness and consistency.

Done when

• [x] Review existing usage
• [x] Review external guidance
• [x] Entry is included in the style guide.

Anything else

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

Proposed entry content

Do not use ampersands. Use the word 'and' instead. This is easier for people to read and understand.