docs: clarify integration example placement

[felix-windsor]

Summary

• add examples/integrations/README.md with placement rules for integration examples, package integrations, custom-function examples, and external community projects
• add a link from .github/CONTRIBUTING.md so contributors can find the guidance before opening integration PRs
• avoid adding any third-party example code directly; this only defines the contribution boundary

Resolves #4744.

Tests

• uv run pre-commit run --all-files

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Summary by cubic

Adds examples/integrations/README.md with placement rules, an example checklist, and a community listing format, and links it from .github/CONTRIBUTING.md. No third-party code added; this only sets the contribution boundary. Resolves #4744.

^{Written for commit 7a33d8d. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}

[sauravpanda]

This is awesome, thanks for the PR