Sub-agents documentation. (#16639)

Co-authored-by: Jenna Inouye <jinouye@google.com>

docs/core/index.md

@@ -7,6 +7,8 @@ requests sent from `packages/cli`. For a general overview of Gemini CLI, see the
 
 ## Navigating this section
 
+- **[Sub-agents (experimental)](./subagents.md):** Learn how to create and use
+  specialized sub-agents for complex tasks.
 - **[Core tools API](./tools-api.md):** Information on how tools are defined,
   registered, and used by the core.
 - **[Memory Import Processor](./memport.md):** Documentation for the modular