Run doc/doap-rendering/update-xeplist.sh to get the latest XEP metadata before building the documentation.
You maybe need to open a local web server to display the list of supported XEPs in case your web browser prohibits the request to doc/doap-rendering/doap.xsl.
Here is an example with Python's built-in web server and Firefox:
python3 -m http.server
firefox http://0.0.0.0:8000/xep.html