Page heading with caption component

[adam-the]

Captions accompanying page headings are a very common pattern, for example, they are used in the mini-hub pattern.

There doesn't seem to be a standard component for this in either the nhsuk-frontend framework or the React library.

I propose one of three implementations:

• As a pattern, similar to [nhsuk-frontend-v5] Move review date component into pattern nhsuk-react-components#138
• Added to Fieldset as Fieldset.Caption and styled/displayed based on the presence of a page heading legend
• As a standalone component in this extensions library

Please let me know which would be preferred and I will submit a PR accordingly.

Initial discussion: NHSDigital/nhsuk-react-components#136 (comment)