Update SUMMARY.md

Author: crStiv

book/src/SUMMARY.md

@@ -27,8 +27,10 @@
 - [Developer Documentation](dev.md)
   - [Contribution Guide](CONTRIBUTING.md)
   - [Design Overview](dev/overview.md)
+  - [Mempool Specification](dev/mempool-specification.md)
   - [Diagrams](dev/diagrams.md)
     - [Network Architecture](dev/diagrams/zebra-network.md)
+    - [Mempool Architecture](dev/diagrams/mempool-architecture.md)
   - [Upgrading the State Database](dev/state-db-upgrades.md)
   - [Zebra versioning and releases](dev/release-process.md)
   - [Continuous Integration](dev/continuous-integration.md)