chore(ui): ad Page Header ux docs

[franzheidl]

Summary

Add Page Header article to ux docs.

Related Issues

Closes #1377

Checklist

• I have performed a self-review of my code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have made corresponding changes to the documentation (if applicable).
• My changes generate no new warnings or errors.
• I have created a changeset for my changes.

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Latest commit: 8ccc757

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[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds UX documentation for the Page Header component and links it from the UX docs table of contents.

Changes:

• Added a new Page Header documentation page describing header regions and usage guidance
• Updated UX contents to include the new Page Header entry

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
docs/ux/page-header.md	New documentation explaining Page Header structure (left identity area, right options area) and guidance for interactive behavior
docs/ux/0_contents.md	Adds the Page Header link under “Page Layout Elements” in the UX docs TOC

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