[docs] Update the versions dropdown to show v6 (#41557)

Author: mnajdova

docs/pages/_app.js

@@ -166,6 +166,10 @@ function AppWrapper(props) {
         metadata: 'MUI Core',
         name: 'Material UI',
         versions: [
+          {
+            text: 'v6 (next)',
+            href: `https://next.mui.com${languagePrefix}/material-ui/getting-started/`,
+          },
           { text: `v${materialPkgJson.version}`, current: true },
           {
             text: 'v4',
@@ -192,6 +196,10 @@ function AppWrapper(props) {
         metadata: 'MUI Core',
         name: 'MUI System',
         versions: [
+          {
+            text: 'v6 (next)',
+            href: `https://next.mui.com${languagePrefix}/system/getting-started/`,
+          },
           { text: `v${systemPkgJson.version}`, current: true },
           { text: 'v4', href: `https://v4.mui.com${languagePrefix}/system/basics/` },
           {