Add docs for API & the agent architecture (#22)

* Add docs for API & the agent
* Add reAct arch diagram

docs/index.md

@@ -1,3 +1,29 @@
-# Geo Assistant
+# Geo Assistant 
 
-A geographic assistant that helps answer questions and perform tasks related to locations and geographic data.
+The Geo Assistant agent is a LangGraph-based [ReAct agent](https://arxiv.org/abs/2210.03629) that can answer questions and perform tasks related to locations and geographic data.
+
+## Architecture
+
+<img src="assets/architecture.png" alt="Architecture" width="100%" style="max-width: 400px;">
+
+The agent follows the ReAct (Reasoning + Acting) pattern, which interleaves reasoning traces and task-specific actions to solve queries. It uses LangGraph to manage state transitions and tool execution.
+
+## Available Tools
+
+The agent has access to the following [tools](tools/index.md):
+
+- **get_place**: Retrieve place information from the Overture Maps database
+- **get_search_area**: Create a buffer area (in km) around a place defined in the agent state
+- **fetch_naip_img**: Fetch NAIP aerial imagery for a given area and date range
+- **summarize_sat_img**: Analyze and summarize satellite image contents using an LLM
+
+## State Management
+
+The agent maintains the following state throughout the conversation:
+
+- **messages**: The conversation history (always updated with each interaction)
+- **place**: Current place feature from Overture Maps (updated by `get_place` tool)
+- **search_area**: Buffer area around the place (updated by `get_search_area` tool)
+- **naip_img_bytes**: NAIP imagery data (updated by `fetch_naip_img` tool)
+
+Tools update the state based on user queries, and these state changes are reflected in the `Streamlit` frontend in real-time.