[EUI+] Adjust styles for EuiHorizontalRule snippets

[acstll]

Summary

Resolves #8144

• adjusted alignment for text in main interactive snippet
• fixed CSS leak affecting margin="none"
• added background color for items in Margin snippet

QA

• in the new site, head to /docs/layout/horizontal-rule/ and manually check the snippets
• make sure code is actually OK

[acstll]

@mgadewoll thanks again for the feedback, I think it's much better now 🙂

[kibanamachine]

Preview staging links for this PR:

• Docs site: https://eui.elastic.co/pr_8326/
• Storybook: https://eui.elastic.co/pr_8326/storybook
• New docs: https://eui.elastic.co/pr_8326/new-docs

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 33fffba

History

• 💚 Build #3450 succeeded ab7f1bb

cc @acstll

[mgadewoll]

Thanks for the updates! 🎉 Changes LGTM 👍