Minor documentaiton improvements

Some notes for doc improvements as I'm going through them.

System prompt

• typo: https://posit-dev.github.io/chatlas/get-started/system-prompt.html#inspiration-and-guides

Structured data

• would be good to know Field comes from pydantic - provide code with imports: https://posit-dev.github.io/chatlas/get-started/structured-data.html#add-descriptions

Async

• need to look into this more, but when running the code in the async section, I get this error:

RuntimeError: asyncio.run() cannot be called from a running event loop
Cell In[54], line 16
     12     async for chunk in stream:
     13         print(chunk)
---> 16 asyncio.run(do_stream("My name is Chatlas."))
Hide Traceback
File [~/.pyenv/versions/3.12.10/lib/python3.12/asyncio/runners.py:191](vscode-file://vscode-app/Applications/Positron.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html#), in run(main, debug, loop_factory)
    161 """Execute the coroutine and return the result.
    162 
    163 This function runs the passed coroutine, taking care of
   (...)
    187     asyncio.run(main())
    188 """
    189 if events._get_running_loop() is not None:
    190     # fail fast with short traceback
--> 191     raise RuntimeError(
    192         "asyncio.run() cannot be called from a running event loop")
    194 with Runner(debug=debug, loop_factory=loop_factory) as runner:
    195     return runner.run(main)