Add example for chat with history

Chat with history is perhaps the most common use case. In fact ollama run works like that. An example with that use case will be great for the newcomers. Here's a sample code:

import ollama

messages = []

def send(chat):
  messages.append(
    {
      'role': 'user',
      'content': chat,
    }
  )
  stream = ollama.chat(model='mistral:instruct', 
    messages=messages,
    stream=True,
  )

  response = ""
  for chunk in stream:
    part = chunk['message']['content']
    print(part, end='', flush=True)
    response = response + part

  messages.append(
    {
      'role': 'assistant',
      'content': response,
    }
  )

  print("")

while True:
    chat = input(">>> ")

    if chat == "/exit":
        break
    elif len(chat) > 0:
        send(chat)

Start by locating the existing documentation or examples in the ollama-python repository and review how current chat examples are organized. Add a Python example based on the supplied chat-with-history flow, and consider the work done when newcomers can see how messages persist across turns and how to exit the interaction.