I don’t think that’s technically possible. Discourse doesn’t have an API discovery route (for example, something similar to the WordPress /wp-json route). The closest thing I’m aware of is the documentation at https://docs.discourse.org/.
For integrations that aren’t covered by the docs, probably the fastest way to figure out if they’re doable is to ask on here about how to achieve some specific goal. The general rule with the API is that anything that can be done through the Discourse UI can also be done via the API.