If you are looking for the online documentation, visit the repo through the URL https://opensees.stairlab.io
git clone https://github.com/stairlab/OpenSeesDocumentation.git
pip install -Ur requirements.txt
make html
on Linux type the following to open a browser with index page
xdg-open ./build/html/index.html
on a Mac
open ./build/html/index.html