[Doc] API

[NiklasVousten]

I think it would be nice to have a short over view over the API endpoints. Maybe in a separate markdown document.

The API documentation can be enabled on the server, but I think it would be nice to have a different source than the own instance of crab-hole.

For example it can be that an instance with API but without documentation is deployed by an admin, and a different member has to query the API. For this person, a short overview over the endpoints would be very helpful. Otherwise this person would have to read the source code, or ask the admin to temporarily enable the documentation.