docs: Backend README에 시스템 아키텍처 추가 - #211

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683, gradle/actions/wrapper-validation@41ad77ca97e46ee2e825d5e6b9a95d8169fcc44c. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/