docs: document algolia option

Author: ulivz

Diff for: docs/config/README.md

@@ -98,6 +98,24 @@ The `serviceWorker` option only handles the service worker. To make your site fu
 Also, only enable this if you are able to deploy your site with SSL, since service worker can only be registered under HTTPs URLs.
 :::
 
+### algolia
+
+- Type: `Object`
+- Default: `undefined`
+
+Using `algolia` option allows you to disable the default built-in headers-based search and leverage the [algolia docsearch](https://github.com/algolia/docsearch), to enable this search, You have to provide at least `apiKey` and `indexName`:
+
+```js
+module.exports = {
+  algolia: {
+    apiKey: '<API_KEY>',
+    indexName: '<INDEX_NAME>'
+  }
+}
+```
+
+Rest available options refer to [docsearch options](https://github.com/algolia/docsearch#docsearch-options). 
+
 ### locales
 
 - Type: `{ [path: string]: Object }`

Diff for: docs/zh/config/README.md

@@ -97,6 +97,24 @@ module.exports = {
 当然，仅仅只在你的网站部署后能用 SSL 的时候开启它，因为 service worker 只能在 HTTPs 的链接下注册。
 :::
 
+### algolia
+
+- 类型: `Object`
+- 默认值: `undefined`
+
+使用 `algolia` 选项可以让你禁用掉默认的基于 headers 的搜索，从而使用 [algolia docsearch](https://github.com/algolia/docsearch)。为了使其生效，你必须提供至少 `apiKey` 和 `indexName` 这两个选项：
+
+```js
+module.exports = {
+  algolia: {
+    apiKey: '<API_KEY>',
+    indexName: '<INDEX_NAME>'
+  }
+}
+```
+
+其他可用的选项可以参考 [docsearch options](https://github.com/algolia/docsearch#docsearch-options)。
+
 ### locales
 
 - 类型: `{ [path: string]: Object }`

Diff for: lib/default-theme/Navbar.vue

@@ -33,7 +33,7 @@ export default {
     },
     isAlgoliaSearch () {
       const algolia = this.algolia
-      return algolia && algolia.appId && algolia.apiKey && algolia.indexName
+      return algolia && algolia.apiKey && algolia.indexName
     },
     isSearch() {
       return !this.isAlgoliaSearch && this.$site.themeConfig.search !== false