Accessibility issue 2 - missing caption element for tables in Props

[karypun]

Pages affected: all component pages
Issue: Missing element for tables within "Props" click to reveal. Found on every components page.
Resolution: Add in caption element

[daniel-ac-martin]

For future reference, this could be fixed by providing a caption prop here: https://github.com/daniel-ac-martin/NotGovUK/blob/master/lib/docs-components/src/Props.tsx#L41

In the GDS system they use the caption, "Primary options".