I’m not the best source of information about this, but the only maintainable way I’m aware of to add a new page to Discourse is to create a plugin that adds a route. It it was my site, I’d try to find another way of solving the problem. If you want to pursue it, have a look at this 6 part guide: Developing Discourse Plugins - Part 1 - Create a basic plugin.
This section of the guide demonstrates how to create a route: Developing Discourse Plugins - Part 5 - Add an admin interface. Note that the example code is creating an admin route. Regular users will not be able to access it. That type of page could be useful for allowing admins to configure a plugin.