docs: Migration and Upgrade Guides (#1487)

www/docs/sidebars.js

@@ -204,6 +204,23 @@ module.exports = {
               type: "doc",
               id: "guides/carts-in-medusa",
             },
+            {
+              type: "doc",
+              id: "advanced/backend/migrations",
+              label: "Migrations"
+            },
+            {
+              type: "category",
+              label: 'Upgrade Guides',
+              collapsed: true,
+              items: [
+                {
+                  type: "doc",
+                  id: "advanced/backend/upgrade-guides/1-3-1",
+                  label: "v1.3.1"
+                },
+              ]
+            },
           ]
         }
       ]