docs: update migration guide (#24835)

Move to its own section in the sidebar.

docs/sidebars.js

@@ -92,6 +92,18 @@ module.exports = {
             className: 'hidden',
           }],
         },
+        {
+          type: "category",
+          label: "Migrating to LCEL",
+          link: {type: 'doc', id: 'versions/migrating_chains/index'},
+          collapsible: false,
+          collapsed: false,
+          items: [{
+            type: 'autogenerated',
+            dirName: 'versions/migrating_chains',
+            className: 'hidden',
+          }],
+        },
       ],
     },
     "security"