Add an example about dialogs

[jtpio]

It would be nice to have an example showing how to use dialogs and / or all the different types of dialogs available.

Some examples already use dialogs, for example:

extension-examples/context-menu/src/index.ts

Lines 25 to 29 in 36a6595

	showDialog({
	title: file.name,
	body: 'Path: ' + file.path,
	buttons: [Dialog.okButton()]
	}).catch(e => console.log(e));

But it could be useful to have a dedicated example so it's easier to find and reference elsewhere.

[jtpio]

Another question that is sometimes asked is how to dismiss a dialog automatically.

For JupyterLab 3 the following snippet seems to be doing the trick (for dismissing the first dialog only):

const hideFirstDialog = async (_: unknown, w: Dialog<unknown>) => {
  Dialog.tracker.widgetAdded.disconnect(hideFirstDialog);
  requestAnimationFrame(() => w.dispose());
};
Dialog.tracker.widgetAdded.connect(hideFirstDialog);

In JupyterLab 4 there is now a ready promise, which could likely be used instead of the requestAnimationFrame above:

https://github.com/jupyterlab/jupyterlab/blob/0f178d3094f07f4474dc171a9493effbf0b38e90/packages/apputils/src/dialog.tsx#L147-L152

[krassowski]

Alternatively, we could improve the documentation which already has a few examples:
https://jupyterlab.readthedocs.io/en/latest/extension/ui_helpers.html