There is no documentation for the remote API available. But you
can find examples in the remote_client/examples/ subfolder
of the ispconfig tar.gz file. A full list of all available functions
can be found in the file /usr/local/ispconfig/interface/lib/classes/remoting.inc.php
Next, I would like to create a site. I browsed the source and found interface/lib/classes/remoting.inc.php and found sites_web_domain_add. I don't know if this is the function or how to call it because I don't know what to fill in for the $params array.
The variables in the $params array are named like the columns in the corresponding database table. You can find also some examples and discussions here in the dev forum about that.