After preparing your Windows or Linux environment, install Spin and Ninja:
pip install spin ninja
Then install the documentation-related dependencies:
pip install -r util/readthedocs/requirements.txt
Then tell Spin to build the Sphinx documentation:
spin docs
Then open a webserver to serve the built HTML files:
python -m http.server -d doc/build/html 8000
And open your local docs in a web browser by visiting http://localhost:8000/.
If it's your first time building the docs, it will take a while but should go faster on subsequent re-builds.