Improve duotone support in River

[rezrah]

Summary

Closes https://github.com/github/primer/issues/3654

• Docs preview
• River Storybook example 1
• River Storybook example 2
• RiverStoryScroll example

A dutotone river example already exists in Storybook, but it's too small and used <strong> element for highlighting, which is now deprecated for accessibility reasons.

This PR replaces those examples with <b> elements, which are more semantically correct and makes a few other adjustments to increase visibility of this feature.

List of notable changes:

• Added <b> tag support to existing duotone river
• Updated font-weight to be semibold, matching all other emphasised text instances
• Increased sizes in our examples to 300.
  I haven't made this part of the API because the size is likely to change depending on usage.
• Added a new dedicated story for Duotone
• Updated RiverStoryScroll examples to use dutotone text, matching the original request
• Added a duotone example to documentation page
• Replaced remote placeholders in River page with a local fixture, as they are currently failing

What should reviewers focus on?

• Happy decreasing font-weight to semibold from browser default of bold (700)?
• Has duotone been applied per design specs

Steps to test:

• Docs preview
• River Storybook example 1
• River Storybook example 2
• RiverStoryScroll example

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:

Please try to provide before and after screenshots or videos

Before	After

[changeset-bot]

🦋 Changeset detected

Latest commit: d2c0684

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

This PR includes changesets to release 6 packages

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

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[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.

[simmonsjenna]

👍 LGTM