Notion already let you add a custom subdomain to your notion.site
domain. To read more about that follow this guide How to change your Notion subdomain
If you instead want to add a custom domain like docs.example.com
then follow this guide
Notaku let you create a full website from a Notion page, one of its feature is also adding a custom domain.
To quickly get started with Notaku
Sing up at notaku.so
Create a website with simple layout
Paste your Notion page (must be set to public)
Add a custom domain from the domains tab
Now you have an awesome new website from your Notion page!