Doc update: Returned data sections

[dshean]

https://slideruleearth.io/web/rtd/user_guide/ICESat-2.html#returned-data

These should be updated to match the current v4 structure of the returned GeoDataFrame objects. We index on time, which is converted to a Pandas Timestamp object. The geometry column stores (longitude, latitude), and these fields are removed. Order of fields listed in doc should be consistent with the actual order.

Should add ancillary fields to the returned data section.

Should also be clear in this sesction that data can be returned different formats, as doc right now mostly focuses on GeoDataFrames. Esp if we start supporting additional output formats like COPC.