This is the source for generating the website: https://snowballstem.org/
You'll need the Perl Template Toolkit, neato from graphviz, the Closure Compiler, pngcrush and pygments - on Debian you can install these with:
apt-get install libtemplate-perl graphviz closure-compiler pngcrush python-pygments
You need the pygments-snowball repo as a subdirectory, for syntax highlighting Snowball code on the website:
git clone https://github.com/hajimes/pygments-snowball.git
You also need the snowball repo in a sibling directory to get warnings if the Snowball code here under code/ is out of date:
(cd .. && git clone https://github.com/snowballstem/snowball.git)
Run:
./generate
This will create the website in ../snowballstem.github.io.