feat($theme-default): allow optional subtitle (#1981)

Author: Sun Haoran

packages/@vuepress/theme-default/components/Home.vue

@@ -9,7 +9,7 @@
 
       <h1 v-if="data.heroText !== null" id="main-title">{{ data.heroText || $title || 'Hello' }}</h1>
 
-      <p class="description">
+      <p v-if="data.tagline !== null" class="description">
         {{ data.tagline || $description || 'Welcome to your VuePress site' }}
       </p>
 

packages/docs/docs/theme/default-theme-config.md

@@ -29,6 +29,8 @@ footer: MIT Licensed | Copyright © 2018-present Evan You
 ---
 ```
 
+You can disable `title` and `subtitle` by setting the corresponding field to `null`.
+
 Any extra content after the `YAML front matter` will be parsed as normal Markdown and rendered after the features section.
 
 To use a fully custom homepage layout, you can also use a [Custom Layout](#custom-layout-for-specific-pages).

packages/docs/docs/zh/theme/default-theme-config.md

@@ -27,6 +27,8 @@ footer: MIT Licensed | Copyright © 2018-present Evan You
 ---
 ```
 
+你可以将相应的内容设置为 `null` 来禁用标题和副标题。
+
 任何 `YAML front matter` 之后额外的内容将会以普通的 markdown 被渲染，并插入到 `features` 的后面。
 
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