docs(install): cover Docker and native deployments

deploy/native/magent-backend.env.example

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+# Copy to /etc/magent/backend.env, owned by root with mode 0600.
+# Generate independent secrets; these placeholders must never be deployed.
+JWT_SECRET=REPLACE_WITH_INDEPENDENT_RANDOM_TOKEN
+SETTINGS_ENCRYPTION_KEY=REPLACE_WITH_FERNET_KEY
+SETUP_TOKEN=REPLACE_WITH_ANOTHER_RANDOM_TOKEN
+MAGENT_MANAGED_SECRETS=false
+MAGENT_APPLICATION_URL=https://magent.example.com
+CORS_ALLOW_ORIGIN=https://magent.example.com
+AUTH_COOKIE_SECURE=true
+API_DOCS_ENABLED=false
+SQLITE_PATH=/var/lib/magent/data/magent.db
+LOG_FILE=/var/lib/magent/data/magent.log
+BRANDING_SOURCE=data