docs: added express checkout guide (#10810)

* initial draft

* docs: add express checkout guide

* fix lint errors

* small fixes

* add screenshot in summary

* add meta image

www/apps/resources/generated/files-map.mjs

@@ -1091,6 +1091,10 @@ export const filesMap = [
     "filePath": "/www/apps/resources/app/storefront-development/customers/third-party-login/page.mdx",
     "pathname": "/storefront-development/customers/third-party-login"
   },
+  {
+    "filePath": "/www/apps/resources/app/storefront-development/guides/express-checkout/page.mdx",
+    "pathname": "/storefront-development/guides/express-checkout"
+  },
   {
     "filePath": "/www/apps/resources/app/storefront-development/page.mdx",
     "pathname": "/storefront-development"