docs: polish README and docs intro

[ethanj]

What

• Centers and constrains the README OKF banner so it renders cleanly on GitHub.
• Adds the same OKF banner to the Mintlify introduction page.
• Adds a top-level docs description so generated agent-facing docs indexes have a useful site summary.
• Updates README contributing copy to welcome need-driven issues and PRs as well as roadmap-driven contributions.

Validation

• npx tsc --noEmit
• npm run build
• npm test
• npx fallow
• npm run release:check-docs:current

[mintlify]

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Project	Status	Preview	Updated (UTC)
llmwiki	🟢 Ready	View Preview	Jun 15, 2026, 12:31 AM

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