The latest documentation can be found at: http://circuitpython.readthedocs.io/en/latest/
The documentation you see there is generated from the files in the whole tree: https://github.com/adafruit/circuitpython/tree/master
If you're making changes to the documentation, you should build the documentation locally so that you can preview your changes.
Install Sphinx, recommonmark, and optionally (for the RTD-styling), sphinx_rtd_theme, preferably in a virtualenv:
pip install sphinx
pip install recommonmark
pip install sphinx_rtd_theme
In circuitpython/, build the docs:
sphinx-build -v -b html . _build/html
You'll find the index page at _build/html/index.html.