Improve accessibility by not skipping a heading level for contributor/signature counts

[jmp]

Description

This fixes a minor accessibility issue with skipped headings on the main page: The heading level jumps from level 2 straight to level 4 in the contributor and signature lists. This goes against accessibility best practices, for example according to W3C:

Skipping heading ranks can be confusing and should be avoided where possible: Make sure that a <h2> is not followed directly by an <h4>, for example.
^{https://www.w3.org/WAI/tutorials/page-structure/headings/#heading-ranks}

The fix is to simply use heading level 3 (instead of level 4) for the contributor/signature counts.

After this change, the contributor and signature count font becomes slightly bigger. But it doesn’t look too bad, at least in my opinion:

Before:

After:

Type of change

This only changes the presentation of the contributor/signature counts. There are no changes to the manifesto text itself.

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Lighthouse	1 paths audited Performance: 74 Accessibility: 82 Best Practices: 100 SEO: 83 PWA: - View the detailed breakdown and full score reports

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[bdfinst]

Thanks for the catch. I'm adding you as a contributor as well.