docs: updates to billing and usage guides (#13222)

* docs: updates to billing and usage guides

* address comments

* fix build error

* add note on billing questions

* Update www/apps/cloud/app/billing/page.mdx

Co-authored-by: Oli Juhl <59018053+olivermrbl@users.noreply.github.com>

---------

Co-authored-by: Oli Juhl <59018053+olivermrbl@users.noreply.github.com>

www/apps/cloud/next.config.mjs

@@ -140,6 +140,11 @@ const nextConfig = {
       destination: "/faq",
       permanent: true,
     },
+    {
+      source: "/billing-usage",
+      destination: "/billing",
+      permanent: true,
+    },
   ],
 }