Organizing the docs API into sections

[quentinblampey]

The API from the docs could be organized into multiple sections for clarity and readability.

For instance, we could have the following sections: "SpatialData object", "Transformations", "Subsampling", "Models", ...
For each sections, there will be a right nav bar to access easily any function within the section, as in pandas (https://pandas.pydata.org/docs/reference/index.html).

[LucaMarconato]

Closed with #785 from @quentinblampey