[Style guide] Add guidance on postcode / zip code / postal code

[alexhallspringer]

What

Develop and provide guidelines on how and when to use postcode/zip code/postal code.

Why

Different teams are using different terms in entry field labels in checkout flows.

Done when

• [ ] Review existing usage.
• [ ] Review external guidance.
• [ ] Find out about current localisation process.
• [ ] Draft guidance.
• [ ] Entry is added to the style guide.

Anything else

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

Proposed entry content

postcode UK English.

zip code US English.

postal code Do not use.