docs: Add ApplyConfigurations guide to SUMMARY

Add the new ApplyConfigurations documentation to the book's table of
contents under the Developer Guide section.

Author: VijetaPriya47

docs/book/src/SUMMARY.md

@@ -71,6 +71,7 @@
     - [Developing "core" Cluster API](developer/core/overview.md)
         - [Rapid iterative development with Tilt](developer/core/tilt.md)
         - [Repository Layout](developer/core/repository-layout.md)
+        - [Using ApplyConfigurations for Server-Side Apply](./developer/architecture/apply-configurations.md)
         - [Controllers](./developer/core/controllers/overview.md)
           - [Cluster](./developer/core/controllers/cluster.md)
           - [ClusterTopology](./developer/core/controllers/cluster-topology.md)