add brief description of how partitioned data currently works/doesn't work

[nickreich]

Add some content in the hub structure documentation page that describes how the current data partitioning set-up does and does not work with current hubverse infrastructure. And maybe something about how this is not an active development priority for the hubverse so using this functionality is "as is". We could link to the SMH hub that has done some of this. And also mention that further hubverse development might focus on cloud-based storage without GitHub which would get around GitHub file size limits, which is one of the reasons we've considered this at all.

[matthewcornell]

Here's the partition documentation from hubData's connect_hub():

partitions: a named list specifying the arrow data types of any partitioning column.

Default:

partitions = list(model_id = arrow::utf8()),