docs: edit migration guides (#4504)

platform/docs/docusaurus.config.js

@@ -193,6 +193,12 @@ module.exports = {
             label: 'Help',
             position: 'left',
           },
+          {
+            to: '/migration-guide/3p8-to-3p9/',
+            //activeBaseRegex: '(^/help$)|(/help)',
+            label: '3.9 Migration Guides',
+            position: 'left',
+          },
           {
             type: 'docsVersionDropdown',
             position: 'right',