[New docs] Implement basic design changes to match the custom theme

[tkajtoch]

Summary

Implement basic design changes to match the typography and layout from the design file.

Please note that this design file is not considered final and there may be additional changes needed before releasing the new documentation site.

Acceptance criteria

• The site's header should be adjusted to match the design (link)
  • Version selector and search bar will be implemented separately and should be skipped for now
  • The cheer button icon should redirect to https://github.com/elastic/eui/tree/main/changelogs until the changelog page is implemented