docs: add optional hardening example for downstream servers

[aak204]

Summary

This docs-only change adds a small optional CI hardening example for downstream MCP server repositories.

Why

docs/quickstart.md is a natural place for a short copy-paste workflow example that downstream users can adapt in their own repositories after they already have a runnable server jar in CI.

Notes

• documentation-only change
• optional example for downstream repos
• does not modify this repository's workflows or Java code
• example tool: https://github.com/aak204/MCP-Trust-Kit

[Kehrlann]

Hey @aak204 !

Thanks for reaching out. Our users are free to use any analysis tool they'd like, but we do not plan on recommending particular tools.