docs(security): add secret key management guidance and examples

[alexandreteles]

Summary

• add a new Secret Key Management documentation page covering threat model, key generation requirements, secure loading patterns, and rotation guidance for authpaseto_secret_key
• document storage/retrieval approaches with explicit pros/cons, including environment variables, Fernet envelope encryption, TPM (tpm2-pytss), HashiCorp Vault, cloud secret managers, and Keycloak positioning
• add a runnable-oriented example file at examples/secure_secret_management.py with secure key generation and retrieval helper functions
• wire the new guide into docs navigation and cross-link from the landing page and examples index

Files changed

• docs/configuration/secret-management.md
• examples/secure_secret_management.py
• mkdocs.yml
• docs/index.md
• docs/examples.md

Notes

• no runtime tests were executed (docs/example focused change).

---

Codex Task