docs: add Poszo Next.js Security Headers Starter

[poszothebuilder]

Summary

Adds the free MIT-licensed Poszo Next.js Security Headers Starter to the Templates & Boilerplates section.

The resource includes tested baseline headers, CSP rollout guidance, validation commands, and rollback notes for Next.js deployments.

Verification

• README-only change
• Public repository: https://github.com/poszothebuilder/poszo-nextjs-security-headers
• MIT license included
• No affiliate link or paid requirement