This is the source for generating the website: https://snowballstem.org/
You'll need the Perl IPC::Run module, Perl Template Toolkit, neato from graphviz, the Closure Compiler, pngcrush and pygments - on Debian you can install these with:
apt-get install libipc-run-perl libtemplate-perl graphviz closure-compiler pngcrush python3-pygments
You need pygments >= 2.2 for Snowball support.
You also need the snowball and snowball-data repos sibling directories:
(cd .. && git clone https://github.com/snowballstem/snowball.git) (cd .. && git clone https://github.com/snowballstem/snowball-data.git)
Run:
./generate
This will create the website in ../snowballstem.github.io.