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

[alexhallspringer]

What

New term 'email' to be added to 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

email All one word. No hyphen.