chore: reorganize package integration docs

docs/developer/package-integration.md

...loper/package-integration/package-integration-database.md

.../package-integration/package-integration-documentation.md

...developer/package-integration/package-integration-flux.md

...per/package-integration/package-integration-monitoring.md

...ckage-integration/package-integration-network-policies.md

...loper/package-integration/package-integration-pipeline.md

...age-integration/package-integration-policy-enforcement.md

...r/package-integration/package-integration-service-mesh.md

.../developer/package-integration/package-integration-sso.md

...eloper/package-integration/package-integration-storage.md

...oper/package-integration/package-integration-supported.md

...eloper/package-integration/package-integration-testing.md

...loper/package-integration/package-integration-upstream.md