docs: add cloud billing docs (#12766)

* docs: add cloud billing docs

* fixes

www/apps/cloud/app/environments/page.mdx

@@ -66,6 +66,7 @@ To create a long-lived environment:
 2. In the side window that opens:
     - Enter the name of the environment. For example, `Staging`.
     - Select the branch you want to associate with this environment. For example, `staging`.
+        - If you don't see the branch you want to use, refresh the page to load the latest branches from your repository.
     - Set the custom subdomain for the environment. All environments are subdomains of `medusajs.app`. So, if you set the subdomain to `staging-my-project`, the environment's URL will be `staging-my-project.medusajs.app`.
 3. Once you're done, click the "Create" button.