iOS (or in this case, iPadOS) has a feature called Guided Access. This basically “locks” you into a single application, so that the end user cannot exit it. Without MDM, you can enable Guided Access in the Settings App for a device, then navigate to the website in Safari, save the website as a Web Clip. Launch that web clip. And then start Guided Access while in the Web Clip. This will lock you into that one website.
However, if you’re wanting to do this en masse on a bunch of devices via MDM, you’d do this via Single App Mode. First you’d need to ensure that these devices were fully “Supervised”. This is done by enrolling them in Apple School Manager, or Apple Business Manager (which then points the device to your MDM server and supervises them), or you can “Supervise” a device with Apple Configurator (a standalone macOS application).
Once the device is Supervised, you want to create a configuration profile and use “Single App Mode” to lock the device into that one single app (or since we’re talking about a website, that Web Clip).
Once this is done, the device (or group of devices) should then be locked to that single app/website completely, even after rebooting it. This can be removed though from the MDM server, or Apple Configurator instance that Supervised the device.