fix(types): missing sidebarDepth at default theme config

Author: ulivz

packages/@vuepress/types/src/theme-default.ts

@@ -133,6 +133,12 @@ export type DefaultThemeConfig = {
     | false /* @see https://vuepress.vuejs.org/theme/default-theme-config.html#disabling-the-sidebar */
     | SidebarConfigArray
     | SidebarConfig4Multiple;
+  /**
+   * Sidebar's depth, set to 0 to disable collapsable sidebar links.
+   * 
+   * @default 1
+   */
+  sidebarDepth?: number;
   /**
    * Enable built-in search
    *