Review the Prototype Kit update instructions

[NoraGDS]

What

How users update to a new version of the Prototype Kit is changing in version 13 because of changes to the update script.

Why

Now that changes to the update script make some updates automatic, guidance about how to update a prototype should be reviewed and updated: https://govuk-prototype-kit.herokuapp.com/docs/updating-the-kit

Who

Content designer, developer

Done when

• [x] guidance reviewed by content designer and developer
• [x] updates reviewed by team
• [ ] final guidance agreed

[NoraGDS]

Updated guidance: https://docs.google.com/document/d/1WQu7PWmua-EdW8RSrp1zpOqxeZKCbJGcWVxp75fp-K8/edit#

[NoraGDS]

This is now covered in the epic