docs: added express checkout guide (medusajs#10810)

* initial draft

* docs: add express checkout guide

* fix lint errors

* small fixes

* add screenshot in summary

* add meta image

Author: shahednasser

www/apps/resources/.content.eslintrc.mjs

@@ -156,6 +156,7 @@ export default [
       "@typescript-eslint/no-non-null-asserted-optional-chain": "off",
       "@typescript-eslint/ban-types": "off",
       "@typescript-eslint/no-unused-expressions": "warn",
+      "@typescript-eslint/no-require-imports": "off",
     },
   },
 ]