Design document history better

[pvh]

Our early user tests made it clear that the current design can be really confusing -- we don't have a clear plan for what to do instead yet.

[pvh]

Notes from an earlier user test:
fix doc navigation in vscode
navigation is easy to get lost
right-click tree node -> "Add as root"
vscode recently opened docs?
vscode display "All docs" from ledger
don't expand nested docs
click to preview
save as new root on change or double-click