docs: update default-theme sidebar type (#3715)

Author: 李小控

Diff for: docs/reference/default-theme-config.md

@@ -135,7 +135,7 @@ export default {
 export type Sidebar = SidebarItem[] | SidebarMulti
 
 export interface SidebarMulti {
-  [path: string]: SidebarItem[]
+  [path: string]: SidebarItem[] | { items: SidebarItem[]; base: string }
 }
 
 export type SidebarItem = {
@@ -162,6 +162,19 @@ export type SidebarItem = {
    * If `false`, group is collapsible but expanded by default
    */
   collapsed?: boolean
+
+  /**
+   * Base path for the children items.
+   */
+  base?: string
+
+  /**
+   * Customize text that appears on the footer of previous/next page.
+   */
+  docFooterText?: string
+
+  rel?: string
+  target?: string
 }
 ```