First, build and _install_ ssapy following the Installing SSAPy section of the documentation.
Then, to build the HTML documentation locally, from the docs directory, run
make html(run it twice to generate all files.)
Then open _build/html/index.html to browse the docs locally.
Alternatively, you can run sphinx-autobuild source _build/html to start a server that watches for changes in /docs and regenerates the HTML docs automatically while serving them at http://127.0.0.1:8000/.
Note that if you updated docstrings, you'll need to re-build and re-install ssapy before re-building the docs.