DocsHub: Start custom domain info

docs/docshub/.nav.yml

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 title: DocsHub
 nav:
   - index.md
+  - custom_domain.md

docs/docshub/custom_domain.md

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+# Custom Domain
+To use a custom domain with your documentation (`docs.yoursite.com`) instead of our hosted subdomain (`yourdocs.hosted.docshub.io`), you'll need to set up DNS records as follows:
+
+## For docs that DON'T require a login
+If your documentation is **public** (you don't need a username/password or SSO to access the site), create a `CNAME` record pointing to `cname.docshub.io`, and then set up your custom domain through the DocsHub portal.
+
+## For docs that DO require a login
+If your documentation is **private** (you need a username/password or SSO to access the site), create a `CNAME` record pointing to `authcname.docshub.io`, and then set up your custom domain through the DocsHub portal.