feat(website): add Utilities > CSS Utility Classes page

[weronikaolejniczak]

Summary

Relates to #8190

Added the CSS Utility Classes page to the new documentation site.

Possible improvement in M2: https://github.com/elastic/eui-private/issues/233

QA

Checklist:

• Compare the content between the old docs and the staging.
• Verify that links redirect as expected (internally, within the same tab; externally, in a new tab).
• Verify that examples work as expected.
• Make sure the prop tables is displayed for all relevant component.

[kibanamachine]

Preview staging links for this PR:

• Docs site: https://eui.elastic.co/pr_8370/
• Storybook: https://eui.elastic.co/pr_8370/storybook
• New docs: https://eui.elastic.co/pr_8370/new-docs

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 4be2671

History

• 💚 Build elastic/eui#3536 succeeded 08178ca