docs(PPDSC-2702): updated fieldset stories

[mstuartf]

PPDSC-2702

What

1. Background - why this is needed
2. What did you do

• Story updates
• Ensure custom storybook themes are used in e2e a11y tests

3. What does the reviewers should expect

I have done:

• Written unit tests against changes
• Written functional tests against the component and/or NewsKit site
• Updated relevant documentation

I have tested manually:

• The feature's functionality is working as expected on Chrome, Firefox, Safari and Edge
• The screen reader reads and flows through the elements as expected.
• There are no new errors in the browser console coming from this PR.
• When visual test is not added, it renders correctly on different browsers and mobile viewports (Safari, Firefox, small mobile viewport, tablet)
• The Playground feature is working as expected

Before:

After:

Who should review this PR:

How to test:

[pp-serviceaccount]

You can preview these changes on:
Documentation Site: http://ncu-newskit-docs-pr.s3-website-eu-west-1.amazonaws.com/ppdsc-2702-fieldset-stories
Storybook: http://ncu-newskit-docs-pr.s3-website-eu-west-1.amazonaws.com/ppdsc-2702-fieldset-stories/storybook