Fix lighthouse interstitial page test

[rmccar]

What is the context of this PR?

Fixes: ONSdigital/design-team#218

The headings in the example were not sequential because the details component was being set to heading level 4 when there was only a h1 above it. Removing this was causing the text to go much bigger than we wanted because it was inside a div with the ons-page__body class. I have also updated that class to not change the size of details component text to stop this.

How to review this PR

• Check the lighthouse tests in the github actions and see that the result for the test for the example-survey-interstitial-page example is now 100
• Also run the test locally on this branch in chrome and on the main branch to see that this issue is now fixed

Checklist

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