Misc docs fixes (#988)

* Add search.md to docs index

* Move metrics.md to monitoring/ folder and add monitoring to docs index

Author: Mark90

docs/reference-docs/metrics.md renamed to docs/reference-docs/monitoring/metrics.md

@@ -190,6 +190,9 @@ nav:
                 # - Workflow Lifecycles: reference-docs/workflows/workflow-lifecycles.md
                 - Callbacks: reference-docs/workflows/callbacks.md
           - Websockets: reference-docs/websockets.md
+          - Search: reference-docs/search.md
+          - Monitoring:
+              - Metrics: reference-docs/monitoring/metrics.md
           - Migration guides:
                 - 2.x: migration-guide/2.0.md
                 - 3.x: migration-guide/3.0.md