feat(tutorial): sidebar stuff, new buttons, new split button, per user feedback via linkedin

[jbecke]

1. For each sidebar tab — docs, agents, etc. — add a "+ New" button to the right of the top tabs. It should have the same behavior as pressing c -> {that item}.
2. Add a "New Split [cmdOrCtrl+]" button to the sidebar, to the bottom, just below the "Create" sidebar item.
3. Add a right click menu for each sidebar item. 3 options: Open in new split, Open fullscreen, Open in current split. Last option, of course, is just the same as left-clicking.

[github-actions]

Preview: https://jbecke-vib-feedback-h8ozw9.preview.macro.com/app (a3cd154)