docs: update admin extension docs (#4778)

* docs: update admin extension docs

* eslint fixes

* fix broken links

* fix broken link

* added admin upgrade guide

* fix lint errors

www/docs/sidebars.js

@@ -97,6 +97,11 @@ module.exports = {
       },
       className: "homepage-sidebar-item",
       items: [
+        {
+          type: "doc",
+          label: "Admin Custom Configuration",
+          id: "admin/configuration",
+        },
         {
           type: "doc",
           label: "Admin Widgets",