Mention alternatives to "none of these"

## What

Add a sentence to Checkboxes components to tell service teams to consider alternatives to the 'None of these' component, as it may not be the best option for some questions.

## Why

Community and working group says that it is important to acknowledge that there are several ways of dealing with the 'none of the above' case and we ought to urge designers to consider different options.

## Who needs to know about this

Content designer, Designer

## Further detail

Discussions in the backlog for [Checkboxes](alphagov/govuk-design-system-backlog#37) component.

## Done when

- [ ] Draft sentence
- [ ] 2i with team, check with people concerned
- [ ] Publish

src/components/checkboxes/index.md.njk

@@ -69,6 +69,8 @@ You can add hints to checkbox items to provide additional information about the
 
 When 'none' would be a valid answer, give users the option to check a box to say none of the other options apply to them — without this option, users would have to leave all of the boxes unchecked. Giving users this option also makes sure they do not skip the question by accident.
 
+Remember to [start by asking one question per page](https://design-system.service.gov.uk/patterns/question-pages/#start-by-asking-one-question-per-page). You might be able to remove the need for a 'none' option by asking the user a better question or filtering them out with a ‘guard question’ beforehand. The GOV.UK Service Manual has guidance on [designing good questions](https://www.gov.uk/service-manual/design/designing-good-questions).
+
 Show the ‘none’ option last. Separate it from the other options using a divider. The text is usually the word ‘or’.
 
 Write a label that repeats the key part of the question.