[New docs] Implement documentation content page design changes

[tkajtoch]

Summary

The documentation content page (DocRootLayoutMain and its children) looks slightly different from the classic docusaurus theme. We need to implement the design changes required for it to match the design.

[!NOTE] Please note that some of the changes are (or will be) implemented outside of this issue.

Acceptance criteria

• Document title should be adjusted to match the design
  • The typography of the heading should be updated
  • A divider line should be added
• Edit this page button should be adjusted to match the design
  • Please note this should be an anchor linking to the source file in the EUI github repository
• Previous and Next should be adjusted to match the design (if needed)

Other changes not listed above (e.g., the breadcrumbs) will be implemented separately.