feat(search-plugin): add namespace option (#706)

* feat(search-plugin): add namespace option

* docs(search-plugin): update doc for option namespace

docs/plugins.md

@@ -40,6 +40,10 @@ By default, the hyperlink on the current page is recognized and the content is s
       depth: 2,
 
       hideOtherSidebarContent: false, // whether or not to hide other sidebar content
+
+      // To avoid search index collision
+      // between multiple websites under the same domain
+      namespace: 'website-1',
     }
   }
 </script>