message: add feature flags to summary.md
diff: * [GraphQL API](administering-lagoon/graphql-queries.md)
  * [Role-Based Access Control \(RBAC\)](administering-lagoon/rbac.md)
  +* [Feature Flags](administering-lagoon/feature-flags.md)
  * [Harbor](administering-lagoon/using_harbor/README.md)
  * [Security Scanning](administering-lagoon/using_harbor/security_scanning.md)
  * [Harbor Settings](administering-lagoon/using_harbor/harbor-settings/README.md)
  