Revise README for clarity and project status

[lukasoppermann]

Updated README to reflect project status and usage instructions.

What are you trying to accomplish?

Make sure folks understand the current state of this repo.

What approach did you choose and why?

What should reviewers focus on?

Can these changes ship as is?

• Yes, this PR does not depend on additional changes. 🚢

[changeset-bot]

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Latest commit: 4d71890

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[Copilot]

Pull Request Overview

Updates the README to clearly communicate that the project is in KTLO (Keep The Lights On) mode and directs users to alternative solutions. The changes simplify the header format and consolidate important status warnings.

• Replaced centered HTML formatting with standard markdown formatting
• Added prominent KTLO warning at the top directing users to primer/react and primer/view_components
• Removed outdated documentation warnings and simplified content

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