Re-organize documentation to list features independently of data objects

[rhugonnet]

This PR updates the documentation structure to describe "Features" in specific sections with nice plots, which makes those more visible and accessible (rather than being stacked in the API/example, or object definitions).

The "Geospatial data objects" become a single section, containing the multiple data types, which will allow to easily add the Xarray accessor (and potentially Geopandas accessor) with limited changes (as they will have the exact same features and API as presented in this new documentation).

This PR also documents pixel interpretation in a section of "Referencing" and the new "Configuration" parameters for the first time, and most point cloud features (in preparation for a 0.2 release with the new PointCloud class).

Resolves #574
Resolves #576
Resolves #523

[adehecq]

I'm just thinking. I'm not sure how well nodata are discussed in the current documentation. Maybe we could have a subsection explaining nodata, when and how they are used, and how to handle the mask and update it (with set_mask). Sometimes people get confused on how to handle it.