It really depends on what you’re trying to do. If you’re trying to pull pieces of your discourse instance into an existing website, then take a look at this:
If you’re trying to go the other way, I would challenge you to ask if this is really necessary. By creating links on discourse that go to your site and links on the site that go back to discourse, you’re effectively doing the same thing.
This is exactly what I do and I have people ask how I managed to incorporate something so dynamic and database driven into my static site. 
I don’t. One is on a subdomain with heavy linking back and forth.