Update heading sizes in the Prose component

[rezrah]

Summary

Towards https://github.com/github/brand-and-marketing-epd/issues/671
Resolves #988

This update makes it easier to distinguish between h2 and h3 elements in the Prose component, which were previously too similar in size.

🔗 Preview link

List of notable changes:

• Updated Prose headings

  • h3 reduced from size-500 to size-400
  • h4 reduced from size-400 to size-300
  • h5 reduced from subhead-large to subhead-medium
  • h2 and h6 are unchanged

• Added a h1 to one of our Prose stories

What should reviewers focus on?

Note: Previous suggestion from Office Hours was to increase the h2 to 28px on narrow viewports in Prose. This wasn't feasible, as we'd need to override a specific (narrow) breakpoint in in our preset typescale. The number of overrides required for this would risk a wider divergence between our primary type scale and Prose implementation. We also can't increase the h2 overall type scale position as that will increase tablet and desktop sizes too.

This PR instead updates Prose headings in the other direction. H1 and H2 remain unchanged, and everything H3+ is now smaller. This should also reduce visual regressions on resource pages, as most pages utilize h1-h4 ranges, rarely using h5 or h6.

Steps to test:

1. Refer to these Storybook examples and check if anything looks odd
2. Review the visual diffs, in before/after mode for anything that looks odd

Contributor checklist:

• All new and existing CI checks pass
• Tests prove that the feature works and covers both happy and unhappy paths
• Any drop in coverage, breaking changes or regressions have been documented above
• UI Changes contain new visual snapshots (generated by adding update snapshots label to the PR)
• All developer debugging and non-functional logging has been removed
• Related issues have been referenced in the PR description

Reviewer checklist:

• Check that pull request and proposed changes adhere to our contribution guidelines and code of conduct
• Check that tests prove the feature works and covers both happy and unhappy paths
• Check that there aren't other open Pull Requests for the same update/change

Screenshots:

Test from a case study page on github.com:

Before	After

From Storybook:

Before	After

[changeset-bot]

🦋 Changeset detected

Latest commit: b89e4d3

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 8 packages

Name	Type
@primer/react-brand	Patch
@primer/brand-docs	Patch
@primer/brand-css	Patch
@primer/brand-primitives	Patch
@primer/brand-e2e	Patch
@primer/brand-fonts	Patch
@primer/brand-config	Patch
@primer/brand-storybook	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions]

🟢 No design token changes found

[github-actions]

🟢 No visual differences found

Our visual comparison tests did not find any differences in the UI.

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[danielguillan]

Works great!

[danielguillan]

Works great!

[simmonsjenna]

Looks good 👍

[kellyquesnel]

meep, sorry y'all. i accidentally hit the wrong button... * * sweats profusely * *

[rezrah]

All good @kellyquesnel, it was ready to merge anyway. Thanks for taking a look and approving it.