Install and enable datalad-next for handbook run

[mih]

In preperation for things like #760, we should transition the handbook to use datalad-next. This would need:

• adjusting the appveyor setup to generate the outputs
• update the installation instructions

The latter is simple for pip, but trickier for others. For pip we only need to replace pip install datalad with pip install datalad-next. There is a Debian package for datalad-next now too. But there is no conda package. so for conda we could either package it (but I am personally not familiar), or we install the datalad package as done now, and follow up with a pip install datalad-next.