Docs: user guide examples

[DeflateAwning]

I think the following examples would be awesome as they capture the bulk of the different styles of core operations in this library. Note that this list may not be exhaustive.

1. [exists] Basic loading/construction.
2. Group by
3. Spatial join
4. Example with multiple geometry columns, explaining the difference between the default
5. [parially exists] Example showing the default geometry column name "geometry", and how it can be changed with the configuration