feat: allow 'auto' in defined sidebar (fix vuejs#1252)

Author: bobheadxi

packages/@vuepress/theme-default/util/index.js

@@ -138,6 +138,9 @@ export function resolveSidebarItems (page, regularPath, site, localePath) {
     return []
   } else {
     const { base, config } = resolveMatchingConfig(regularPath, sidebarConfig)
+    if (config === 'auto') {
+      return resolveHeaders(page)
+    }
     return config
       ? config.map(item => resolveItem(item, pages, base))
       : []