dhi guides: securing a backstage portal using DHI (#24359)

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## Description

This PR adds a guide that shows how to secure a Backstage developer
portal container using Docker Hardened Images, covering native module
compilation, multi-stage builds, Socket Firewall protection, and
distroless runtime images

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## Related issues or tickets

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## Reviews

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- [ ] Technical review
- [ ] Editorial review
- [ ] Product review

Author: ajeetraina