docs: improvement to sidebar (#11810)

* docs: improvement to sidebar

* fix sidebar

www/packages/docs-ui/src/providers/Sidebar/index.tsx

@@ -301,7 +301,7 @@ export const SidebarProvider = ({
   }, [scrollableElement, isBrowser])
 
   const activeMainSidebar = useMemo(() => {
-    if (!activePath) {
+    if (!activePath || sidebars.length === 1) {
       // set first sidebar as active
       return sidebars[0]
     }
@@ -357,7 +357,7 @@ export const SidebarProvider = ({
 
   const shownSidebar = useMemo(() => {
     if (!sidebarHistory.length) {
-      return
+      return sidebars.length === 1 ? sidebars[0] : undefined
     }
 
     return getSidebar(sidebarHistory[sidebarHistory.length - 1])