experimental/cell_space: Add module level docstring to __init__.py

[EwoutH]

Currently the cell space docs don't provide a high-level overview. It also looks like __init__.py and it's docstring is not included in Readthedocs.

@quaquel could you pick this up?

Edit: Voronoi is also not included in the API docs.

[quaquel]

I have added both files via #2450. In my view can the overview stuff wait for 3.1 and be done via the migration guide.