docs: Add ACP-related content (#36966)

Release Notes:

- N/A

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>
Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>
Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Matt Miller <mattrx@gmail.com>

docs/src/SUMMARY.md

@@ -47,6 +47,7 @@
 - [Overview](./ai/overview.md)
 - [Agent Panel](./ai/agent-panel.md)
   - [Tools](./ai/tools.md)
+  - [External Agents](./ai/external-agents.md)
 - [Inline Assistant](./ai/inline-assistant.md)
 - [Edit Prediction](./ai/edit-prediction.md)
 - [Text Threads](./ai/text-threads.md)