The link provided on the 'ArcGIS API for Python' page appears to be dead. Did it move?
Like Dan, the link to the API documentation you have in the subject line works for me this morning.
Main page link https://developers.arcgis.com/python/
api link on that page https://esri.github.io/arcgis-python-api/apidoc/html/
working for me
Retrieving data ...