Subdomain sucks for SEO and that's why we switched our Sidemail docs to a subfolder. I highly recommend you do the same, so you're not leaving any SEO juice on the table.
"Moz moved our Beginner's Guide to SEO from guides.moz.com to moz.com itself. Rankings rose immediately, with no other changes. We ranked higher not only for "seo guide" (outranking Google themselves) but also for "beginners guide" a very broad phrase." — the co-founder of Moz (source).
To make the switch, we had to migrate our docs from 3rd party to self-hosted. If you're considering self-hosting your docs as well, I can't recommend enough Netlify — free & fast static hosting with built-in CDN.
Lastly, if you need to reliably deliver superb & bulletproof transactional emails from your application to your users' inboxes via simple HTTP API, check out the new Sidemail docs!