docs: mark a new example repository as a published copy

[arcjet-rei]

Adds a step to "Publishing to a new repository": an organization owner sets the source_repository custom property to arcjet/examples on the new repository and applies the "Derived repo" code security configuration, which turns off its Dependabot alerts.

Each arcjet/example-* repository is overwritten on every publish, so its alerts repeat this repository's and a fix made in it doesn't survive the next publish. The 37 existing copies were marked this way on 2026-10-06; this step keeps new ones consistent.

🤖 Generated with Claude Code

[arcjet-review]

Arcjet Review — 🟢 Low Risk

Decision: Approved

Rationale: This PR only updates CONTRIBUTING.md with an additional procedural step for publishing example repositories. No code, dependencies, infrastructure, authentication, database schema, or executable configuration are changed. The security review checklist found no secrets, injection risks, cryptographic concerns, or access-control changes in the diff.

Summary of Changes

Adds documentation instructing maintainers to mark newly published example repositories as derived copies by setting the source_repository custom property and applying the Derived repo security configuration.

_{Review: 08154af3 | Model: openai/gpt-5.5 | Powered by Arcjet Review}