feat(v2): ability to add/override theme html metadatas (#3406)

* ability to add/override theme html metadatas
see https://github.com/facebook/docusaurus/issues/3024

* refactor/fix validateThemeConfig tests

website/docs/theme-classic.md

@@ -85,6 +85,20 @@ module.exports = {
 };
 ```
 
+### Metadatas
+
+You can configure additional html metadatas (and override existing ones).
+
+```js {4-6} title="docusaurus.config.js"
+module.exports = {
+  // ...
+  themeConfig: {
+    metadatas: [{name: 'twitter:card', content: 'summary'}],
+    // ...
+  },
+};
+```
+
 ### Announcement bar
 
 Sometimes you want to announce something in your website. Just for such a case, you can add an announcement bar. This is a non-fixed and optionally dismissable panel above the navbar.

website/docusaurus.config.js

@@ -228,6 +228,7 @@ module.exports = {
       darkTheme: require('prism-react-renderer/themes/dracula'),
     },
     image: 'img/docusaurus-soc.png',
+    // metadatas: [{name: 'twitter:card', content: 'summary'}],
     gtag: {
       trackingID: 'UA-141789564-1',
     },