To create a website you need a regular Notion page, i.e. the root page cannot be a Notion database.
Here’s the Notion page of this documentation, you can duplicate it as a starting point for your website: https://egoist.notion.site/Documentation-8bdba6758e8a4d968cd12d65d1f112ec
Make sure this page is set to be public in the “Share” menu on Notion:
In the Chelly dashboard, click “New” or use the link here: https://chelly.io/new-site to create your website, paste the page URL you got in the last step:
Hit “Create” and you’re all set.