chore: Improve docs for heading level in Accordion

[VincentSmedinga]

Describe the pull request

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What

• Uses h3 instead of h1 by default in Accordion examples
• Hides the undefined option for headingLevel in the Controls

Why

• Heading level 1 was confusing. One user didn’t realize they could change that level. An Accordion will not use level 1 headings very often – 3 seems like a good default.

How

• Improved Storybook config
• Added a guideline

Checklist

Before submitting your pull request, please ensure you have done the following. Check each checkmark if you have done so or if it wasn't necessary:

• Add or update unit tests
• Add or update documentation
• Add or update stories
• Add or update exports in index.* files
• Start the PR title with a Conventional Commit prefix, as explained here.

Additional notes

• None