Add documentation (#68)

* initial docs

* update docs

* Update agent.md

* Update memory.md

* Update runtime.md

---------

Co-authored-by: Jack Gerrits <jackgerrits@users.noreply.github.com>

docs/src/core-concepts/agent.md

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+# Agent
+
+An agent in AGNext is an entity that can react to, send, and publish
+messages. Messages are the only means through which agents can communicate
+with each others.
+
+## TypeRoutedAgent
+
+`agnext.components.TypeRoutedAgent`
+is a base class for building custom agents. It provides
+a simple API for associating message types with message handlers.
+Here is an example of simple agent that reacts to `TextMessage`:
+
+```python
+from agnext.chat.types import TextMessage
+from agnext.components import TypeRoutedAgent, message_handler
+from agnext.core import AgentRuntime, CancellationToken
+
+class MyAgent(TypeRoutedAgent):
+    
+    @message_handler()
+    async def on_text_message(self, message: TextMessage, cancellation_token: CancellationToken)) -> None:
+        await self._publish(TextMessage(content=f"I received this message: ({message.content}) from {message.source}"))
+```

docs/src/core-concepts/memory.md

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+# Memory
+
+Memory is a collection of data corresponding to the conversation history
+of an agent.
+Data in meory can be just a simple list of all messages, or a sightly more
+realistic one which provides a view of the last N messages
+(`agnext.chat.memory.BufferedChatMemory`).

docs/src/core-concepts/patterns.md

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+# Multi-Agent Patterns

docs/src/core-concepts/runtime.md

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+# Agent Runtime
+
+Agent runtime is the execution environment for agents in AGNext.
+Similar to the runtime environment of a programming language, the
+agent runtime provides the necessary infrastructure to facilitate communication
+between agents, manage agent states, and provide API for monitoring and
+debugging multi-agent interactions.
+
+Further readings:
+
+1. `agnext.core.AgentRuntime`
+2. `agnext.application.SingleThreadedAgentRuntime`

docs/src/core-concepts/tools.md

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+# Tools

docs/src/getting-started/contributing.md

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+# Contributing to AGNext

docs/src/getting-started/tutorial.md

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+# Tutorial

docs/src/index.rst

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     :hidden:
 
     getting-started/installation
+    getting-started/tutorial
+    getting-started/contributing
+
+.. toctree::
+    :caption: Core Concepts
+    :hidden:
+
+    core-concepts/runtime
+    core-concepts/agent
+    core-concepts/patterns
+    core-concepts/memory
+    core-concepts/tools
 
 .. toctree::
     :caption: Guides