[Style guide] Add guidance on register, sign up, create an account usage

[alexhallspringer]

What

Develop and provide guidelines on how and when to use 'sign up', 'register' or 'create an account'.

Why

For completeness and consistency

Done when

• [x] Review existing usage.
• [x] Review external guidance.
• [x] Find out about current localisation process.
• [x] Draft guidance.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