feat(website): add local docs search with centered navbar input

Add @easyops-cn/docusaurus-search-local for offline full-text search
across all docs pages. Center the search bar in the navbar and widen
it to 360px for better usability.

Author: yiwang

website/docusaurus.config.ts

@@ -40,6 +40,16 @@ const config: Config = {
     ],
   ],
 
+  themes: [
+    [
+      '@easyops-cn/docusaurus-search-local',
+      {
+        hashed: true,
+        indexBlog: false,
+      },
+    ],
+  ],
+
   plugins: [
     [
       '@docusaurus/plugin-client-redirects',