feat: extend pass vault guide for secrets

[aadam-shopware]

This pull request adds comprehensive documentation to the secrets-vault-guide.md for Shopware PaaS, clarifying the distinction between system-managed and user-managed secrets, safe handling practices, and maintenance guidelines. It introduces best practices for secret management, details on permissions, and steps for recovery and support, helping users avoid common pitfalls and platform outages.

Secret Management & Ownership:

• Added detailed explanations distinguishing system-managed secrets (critical for platform operation and not user-editable) from user-managed secrets, with a reference table describing common secrets, their purpose, and editability.
• Included clear warnings and best practices to prevent deletion or modification of system-managed secrets, outlining the consequences and escalation steps if issues arise.

Secret Maintenance & Cleanup:

• Provided a recommended process for auditing, backing up, and deleting unused or legacy secrets, including how to handle typo or deprecated secrets.
• Added instructions for filtering secrets by application and maintaining documentation of secret ownership and usage.

Safety, Recovery & Support:

• Outlined safety measures such as regular backups, credential rotation, and limiting access, along with step-by-step guidance for recovering from accidental deletions.
• Documented the escalation process for unresolved issues, including what