cd /path/to/icefall/docs
pip install -r requirements.txt
make clean
make html
cd build/html
python3 -m http.server 8000

It prints:

Serving HTTP on 0.0.0.0 port 8000 (http://0.0.0.0:8000/) ...

Open your browser and go to http://0.0.0.0:8000/ to view the generated documentation.

Done!

Hint: You can change the port number when starting the server.