Define `overrides.json` under `jupyter/private` in SMAI

For private configs (under `template/v4/v4.x/dirs/etc/sagemaker/jupyter/private`), should we also just have the `lab/settings/overrides.json` file defined here too?

The existing one seems to only apply to Jupyter AI, so maybe it should just go under there too:

```
{
"@jupyterlab/application-extension:shell": {
"layout": {
"multiple": {
"JupyterlabChat:sidepanel": {
"area": "right"
}
}
}
},
"@jupyterlab/filebrowser-extension:browser": {
"showHiddenFiles": true
}
}
```

_Originally posted by @dlqqq in https://github.com/aws/sagemaker-distribution/pull/1329#pullrequestreview-5081716784_

Start by inspecting the private configuration directory at template/v4/v4.x/dirs/etc/sagemaker/jupyter/private and the existing lab/settings/overrides.json file. Confirm which settings are specific to Jupyter AI, then define the shown overrides under the appropriate private Jupyter location; done means the configuration is placed consistently and applies the intended JupyterLab layout and hidden-file settings.

Written by the indexing model from the issue text.