feat(sidebar): enhance document management UI and improve layout structure

- Updated the FolderTreeView component to clarify upload instructions.
- Introduced a new renderDocumentTree function in DocumentsSidebar for better organization and readability.
- Refactored the Sidebar component to utilize SidebarSection for improved layout consistency.
- Added contentClassName prop to SidebarSection for flexible styling options.

surfsense_web/components/documents/FolderTreeView.tsx

@@ -336,7 +336,7 @@ export function FolderTreeView({
 			<div className="flex flex-1 flex-col items-center justify-center gap-1 px-4 py-12 text-muted-foreground select-none">
 				<p className="text-sm font-medium">No documents found</p>
 				<p className="text-xs text-muted-foreground/70">
-					Use the upload button or connect a source above
+					Use the plus menu to upload files or manage connectors
 				</p>
 			</div>
 		);