This is discourse multisite no?
There used to be another one titled “running other websites on same machine as discourse” but I can’t seem to find it.
The process overall is straightforward:
Stop discourse,
Install a reverse proxy like nginx
Edit the discourse and reverse proxy configuration
Start discourse
Now if everything goes as planned, the reverse proxy should be serving discourse.
You can create new sites using the same reverse proxy as webserver.
But one word of caution, the Pi is under powered, it might not be able to sustain the load of multiple websites.