Update Jupyternaut configuration examples

[uuzzrm]

Description

Fixes #1593.

The Jupyternaut configuration page still used the old AiExtension prefix, so the documented command-line and JSON examples did not target the extension that owns these traits. This updates the examples to JupyternautExtension and removes the obsolete default_max_chat_history example, which is no longer part of the extension configuration.

Code changes

• Update the documented traitlets prefix for Jupyternaut settings.
• Remove the no-longer-supported chat history setting example.

User-facing changes

Users following the configuration guide will now get the correct extension settings when configuring Jupyternaut at startup.

[srdas]

@uuzzrm Thanks for updating this documentation. Some stray references to the old AiExtension name still exist in jupyter-ai-jupyternaut/conftest.py and secrets_manager.py (docstrings/comments only, not the docs), so there's a small follow-up cleanup opportunity there, if you'd like to do that as well.

I can wait to merge this if you'd like to update the PR.