Expand documentation of fieldset to describe how to have errors and hints

[romaricpascal]

What

Add example of how to render errors and hints inside a Fieldset component, especially:

• the markup for the errors (the Error message component can help with that) and the hint (the Hint component is private, so that'll call for a discussion on what to do there)
• applying correct styles to the fieldset when an error happens (the Date input can provide an example of how to achieve this)
• how to accessibly link the error and hint to the fieldset using aria-describedby (similarly the Date input might help with that)

Why

This would help people implement inputs split across multiple fields in a similar fashion as the Date input. Thanks @edwardhorsford for bringing this to our attention.

Who needs to work on this

Developers, Tech writers

Who needs to review this

Developers, Tech writers

Done when

• The Fieldset component documentation explains how to have errors and hints
• [Stretch] we've discussed whether to abstract rendering the errors and hint as part of the component options

[romaricpascal]

This overlaps with: alphagov/govuk-frontend#1166

[edwardhorsford]

Haha past Ed strikes again!