docs: add sidebar link for load testing madr

Author: spwoodcock

mkdocs.yml

@@ -153,6 +153,7 @@ nav:
           - Backend dependencies: decisions/0008-uv.md
           - API Documentation: decisions/0002-mkdocs.md
           - Technical Diagrams: decisions/0001-draw-io.md
+          - Load Testing: decisions/0009-load-testing.md
   - DevOps:
       - Reusable Workflows: https://hotosm.github.io/gh-workflows/
       - Kubernetes: https://github.com/hotosm/k8s-infra