docs: update Agent documentation (#1333)

Summary:
- [new] Agent concepts (session, turn)
- [new] how to write custom tools
- [new] non-streaming API and how to get outputs
- [update] remaining `memory` -> `rag` rename
- [new] note importance of `instructions`

Test Plan:
read

docs/source/building_applications/index.md

@@ -8,22 +8,24 @@ The best way to get started is to look at this notebook which walks through the
 
 Here are some key topics that will help you build effective agents:
 
-- **[Agent Execution Loop](agent_execution_loop)**
-- **[RAG](rag)**
-- **[Safety](safety)**
-- **[Tools](tools)**
-- **[Telemetry](telemetry)**
-- **[Evals](evals)**
-
+- **[Agent](agent)**: Understand the components and design patterns of the Llama Stack agent framework.
+- **[Agent Execution Loop](agent_execution_loop)**: Understand how agents process information, make decisions, and execute actions in a continuous loop.
+- **[RAG (Retrieval-Augmented Generation)](rag)**: Learn how to enhance your agents with external knowledge through retrieval mechanisms.
+- **[Tools](tools)**: Extend your agents' capabilities by integrating with external tools and APIs.
+- **[Evals](evals)**: Evaluate your agents' effectiveness and identify areas for improvement.
+- **[Telemetry](telemetry)**: Monitor and analyze your agents' performance and behavior.
+- **[Safety](safety)**: Implement guardrails and safety measures to ensure responsible AI behavior.
 
 ```{toctree}
 :hidden:
 :maxdepth: 1
 
+agent
 agent_execution_loop
 rag
-safety
 tools
 telemetry
 evals
+advanced_agent_patterns
+safety
 ```