The best way to do it is having this outside the Mendix folders or hosted on an external service.
You can also store the html in a string attribute of an entity and then render it on a page. Set the access rules such that only an administrator can update this html.
Create a page where the admin can write and update the html in a text area.
Hope this helps.
You will find it easier to create and manage any HTML content in your application directly in Studio Pro.
If the content is essentially static, then you can use the HTML Snippet widget to add custom HTML to a Mendx page.
If you need something a bit more dynamic, then you can use the HTML Element widget.