The API has to do two things: perform an OAUTH authentication, and respond to API queries. Slack uses OAUTH to authenticate that a user really does want to add the app to the Slack team. In essence, this happens in three steps. Slack sends a web request to your API, the API server then requests an OAUTH token from the Slack service (using a secret key), and finally returns this token on the original Slack connection.
As for serving up the T-SQL reference documentation, it’s just a matter of opening static text files in the web server directory and passing them with some encoding to the client. That’s it.
All of these exchanges are JSON encoded, which is trivial to work with in node.js.
If you’re spending all day in Slack, this seems like a pretty good thing to do.