docs: added publish plugin documentation (#3137)

www/docs/sidebars.js

@@ -399,6 +399,11 @@ module.exports = {
           id: "advanced/backend/plugins/create",
           label: "Create a Plugin"
         },
+        {
+          type: "doc",
+          id: "advanced/backend/plugins/publish",
+          label: "Publish a Plugin"
+        },
         {
           type: "doc",
           id: "advanced/backend/migrations/index",