[EuiPageHeader] Style updates

[ryankeairns]

Related to #8425 (comment)

Important

This will be shared as a POC prior to review/merge

Summary

The now-previous theme, Amsterdam, brought a larger base font size with noticeably heavy titles. Some of these aspects were baked directly into the EuiPageHeader.

1. Tone down some hallmark elements of Amsterdam
2. Fix spacing between right side items

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Changes

For item 1

• Decrease title size (and associated icon)
• Decrease description size
• Decrease tabs size

For item 2

• Gutter size reduced on right-side items to match the original design as long seen in the Figma component.

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EUI docs

Note

The current EUI docs site uses the page header component.

This PR contains a style override that preserves visual hierarchy of headings in the EUI docs site. The style override results in no visual changes to longtime users of the current docs site (i.e. title font size remains 30px).

Alternatively, I considered reworking all the 'guidelines' titles - downsizing each - but decided against this given the imminent switchover to EUI+.

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XL tabs

Note

It is likely we will deprecate/downsize xl tabs, separately.

QA

Remove or strikethrough items that do not apply to your PR.

General checklist

• Browser QA
  • Checked in both light and dark modes
  • Checked in mobile
  • Checked in Chrome, Safari, Edge, and Firefox
  • Checked for accessibility including keyboard-only and screenreader modes
• Docs site QA
  • Added documentation
  • Props have proper autodocs (using @default if default values are missing) and playground toggles
  • Checked Code Sandbox works for any docs examples
• Code quality checklist
  • Added or updated jest and cypress tests
  • Updated visual regression tests
• Release checklist
  • A changelog entry exists and is marked appropriately.
  • If applicable, added the breaking change issue label (and filled out the breaking change checklist)
• Designer checklist
  • If applicable, file an issue to update EUI's Figma library with any corresponding UI changes. (This is an internal repo, if you are external to Elastic, ask a maintainer to submit this request)

[kibanamachine]

Preview staging links for this PR:

• Docs site: https://eui.elastic.co/pr_8445/
• Storybook: https://eui.elastic.co/pr_8445/storybook
• New docs: https://eui.elastic.co/pr_8445/new-docs

[elasticmachine]

💚 Build Succeeded

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• Commit: 4b52ac1

[ryankeairns]

Fixed the gap here; will address page title heading separately.