How can I serve a static file in response to a Webfinger query?

I am trying to follow this ActivityPub tutorial.

I would like to get my server to respond with a static file when it gets a query for https://domain.com/.well_known/webfinger?resource=act:username@domain.com

Since it is not explained in the tutorial I suppose that this is a very basic thing to do, but I have never dealt with this type of query before. What is the simplest way achieve this? My server is using nginx to serve a basic static html at domain.com.