Add guidance on alpha and beta banners
Add guidance on alpha and beta banners

Content is copied from:
https://www.gov.uk/service-manual/user-centred-design/resources/patterns
/alpha-beta

TFOH/govuk_elements,gavboulton/govuk_elements,alphagov/govuk_elements,alphagov/govuk_elements,gavboulton/govuk_elements,alphagov/govuk_elements,joelanman/govuk_elements,TFOH/govuk_elements,gavboulton/govuk_elements,gavboulton/govuk_elements,alphagov/govuk_elements,joelanman/govuk_elements