[Site refactor] Update Testimonial documentation

[josepmartins]

Summary

Updates the design guidelines regarding the site refactor new UI and layout adjustments

Fixes: https://github.com/github/primer/issues/2706
Part of: https://github.com/github/primer/issues/2620

List of notable changes:

• updated images in Testimonial design guidelines
• added emphasis usage examples

What should reviewers focus on?

Design guidelines: https://primer-1543f44201-26139705.drafts.github.io/components/Testimonial

Supporting resources (related issues, external links, etc):

• https://github.com/github/primer/issues/2620
• https://github.com/github/primer/issues/2706
• https://github.com/github/primer/issues/2506
• [Testimonial] Site refactor updates #424
• 2023 refactor for the large Testimonial #372

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
• New visual snapshots have been generated / updated for any UI changes
• 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]

⚠️ No Changeset found

Latest commit: 7582fb2

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[github-actions]

🟢 No design token changes found

[github-actions]

⚠️ Visual differences found

Our visual comparison tests found UI differences.

Please review the differences by using the test artifacts to ensure that the changes were intentional.

Artifacts can be downloaded and reviewed locally.

Download links are available at the bottom of the workflow summary screen.

Example:

If the changes are expected, please run npm run test:visual:update-snapshots to replace the previous fixtures.

Review visual differences

[rezrah]

👍 LGTM