Disable save commands due to autosaving to avoid user confusion

[ellisonbg]

This commit disables JupyterLab's manual save commands since documents are now autosaved. The implementation:

• Overrides four save-related commands: save, save-as, save-all, and toggle-autosave
• Removes keyboard shortcuts: Cmd/Ctrl+S and Cmd/Ctrl+Shift+S no longer trigger save operations
• Provides user feedback: Menu items show "(Autosaving)" labels and tooltips explaining save operations are disabled
• Maintains UX consistency: Commands remain visible but execute as no-ops instead of being completely hidden

The plugin activates after app restoration to ensure proper command registry manipulation using JupyterLab's private API.

[dlqqq]

Looks great! Since this overrides commands that are unused anyways, this looks safe. I will test this more thoroughly tomorrow, before we do a release ideally.

One optional suggestion: the benefit of this approach is that we can define what happens when a command is attempted. Do you think it's worth showing a small notification that displays "documents are autosaved" when a user attempts to save manually?

EDIT: Tested and verified that this works locally. 👍

[dlqqq]

@ellisonbg PR that adds some minor enhancements to this branch: https://github.com/ellisonbg/jupyter-server-documents/pull/1

[dlqqq]

Also opened a follow-up issue: #128

[dlqqq]

Any chance we could show the notification again after a certain interval of time instead of a certain number of attempts? Worried users could miss the first notification and believe the app to be broken.

A third approach would be to just create a notification every time a save is attempted. That also encourages the user to stop manually saving to avoid a flood of notifications.

[dlqqq]

Approving to unblock. Opened a follow-up issue: #129.