Add a keyboard shortcut for duplicating blocks in the document editor and show that shortcut in the block action menu.
### Description **Summary** Add a keyboard shortcut for duplicating blocks in the document editor and show that shortcut in the block action menu. **Current behavior** The document block action menu has a Duplicate action, but it can only be triggered from the menu. There is no keyboard shortcut for it. **Expected behavior** Cmd + D duplicates the current block on macOS Ctrl + D duplicates the current block on Windows/Linux If multiple blocks are selected, the shortcut duplicates all selected blocks The block action menu shows the shortcut next to Duplicate The shortcut appears in Settings > Shortcuts ### Impact This makes duplication faster for desktop users and keeps the editor more consistent with other shortcut-driven actions. ### Additional Context _No response_