Add dedicated general security guidance to Shopware security reference

[Copilot]

This updates the Shopware security reference to serve as a dedicated, high-signal security page. It adds explicit guidance for patch upgrades vs. Security Plugin usage, a central vulnerability tracking entry, and baseline hardening recommendations.

• Security update strategy

  • Added a new Security updates section in resources/references/security.md.
  • Clarifies: upgrade to the latest patch version first; use the Security Plugin as interim mitigation if immediate upgrade is not possible.

• Published vulnerability visibility

  • Added a Published vulnerabilities section with a table linking to published advisories:
    • https://github.com/shopware/shopware/security/advisories?state=published
  • Makes advisory tracking discoverable from the main security page.

• Baseline hardening guidance

  • Added a Basic hardening checklist covering:
    • strong/unique passwords,
    • APP_ENV=prod for public production systems,
    • restricting staging/preview environments from public internet access.

• Link correctness

  • Updated the existing Security Plugin reference link to point to the dedicated guide:
    • ../../guides/hosting/installation-updates/security-plugin.md

## Basic hardening checklist

- Use strong, unique passwords for Administration users, database users, and all infrastructure accounts.
- Set `APP_ENV=prod` in production environments. Do not run publicly reachable production systems with `APP_ENV=dev`.
- Keep staging or preview environments private. Protect them with authentication, IP allowlists, or VPN access and do not expose them publicly on the internet.

• Fixes Create General Security Page #2325

[shopware-dev-docs-connector]

Developer Docs healthcheck

Status: Completed with success.
Repository: shopware/docs
Commit: 225b017
Preview: https://developer-documentation-9gwtaygpt-shopware-frontends.vercel.app
Workflow run: #4635