Add missing TOC link to poetry cheat sheet (#13022)

Missed by #12475.

changelog.d/13022.doc

@@ -0,0 +1 @@
+Ensure the [Poetry cheat sheet](https://matrix-org.github.io/synapse/develop/development/dependencies.html) is available in the online documentation.

docs/SUMMARY.md

@@ -88,6 +88,7 @@
   - [OpenTracing](opentracing.md)
   - [Database Schemas](development/database_schema.md)
   - [Experimental features](development/experimental_features.md)
+  - [Dependency management](development/dependencies.md)
   - [Synapse Architecture]()
     - [Cancellation](development/synapse_architecture/cancellation.md)
     - [Log Contexts](log_contexts.md)