At long last we have a mechanism to publish and provide access to multiple versions of ParaView’s C++ and Python API docs.
The new URL is https://kitware.github.io/paraview-docs/
This should take you to the latest C++ docs for the active development branch.
You can pick a different version (currently 5.6.0-RC and 5.5.2 are available) using the version drop down on the top-left or switch between C++ and Python docs using the icons on the top right. Similar version selection and language switching capabilities are provided in the Python docs as well.
Old URLs will soon start forwarding to https://kitware.github.io/paraview-docs/nightly/ as well.