docs: added doc pages for v1.20.4 (#6828)

- Added new documentation page for Medusa Worker.
- Updated Railway deployment guide with details on Medusa Worker.
- Small adjustments to the `worker_mode` option's TSDocs.

www/apps/docs/sidebars.js

@@ -1969,6 +1969,11 @@ module.exports = {
             },
           ],
         },
+        {
+          type: "doc",
+          id: "development/worker-mode/index",
+          label: "Worker Mode",
+        },
         {
           type: "category",
           label: "Feature Flag",