Refresh UI customization docs from guide

[yoen-velt]

Summary

• Replace the UI customization docs with the downloaded guide content while preserving the docs site route/layout conventions.
• Flatten the old nested UI customization feature pages into guide, approach, feature, guidance, and reference sections.
• Add redirects and update cross-links from removed legacy UI customization pages to the new guide/reference pages.
• Add an empty public/custom.css so local Mintlify preview can start cleanly.

Review / validation

• Confirmed all 52 non-template guide pages are mapped into public docs pages, plus README.md -> /ui-customization/overview and _entry-contract.md -> /ui-customization/reference/source-of-truth-invariant.
• Checked docs.json: 389 nav pages and 194 redirects have valid destination files.
• Ran git diff --check.
• Restarted mint dev cleanly.
• Crawled localhost preview: 389 rendered docs pages and 415 unique internal rendered anchor targets all returned 2xx/3xx.

[mintlify]

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Project	Status	Preview	Updated (UTC)
velt	🟢 Ready	View Preview	Jun 29, 2026, 11:39 PM