astroquery.ibe's query_region_async is non-functional

The URL constructed for querying IRSA is incorrect. In the process of fixing this bug, I also modified the file to make clear that the simple image access (SIA) API at URSA is different enough that it really deserves a separate wrapper. The IRSA's different data fetching APIs are documented [here](https://irsa.ipac.caltech.edu/ibe/). The SIA API only grants access to a small subset of what's available from IBE, and the IBE cutout facility is not available for everything in the SIA API. As it stands, the astroquery IBE uses [IRSA's TAP](https://irsa.ipac.caltech.edu/docs/program_interface/TAP.html) interface to fetch image metadata. As explained in the file, image fetching is not yet implemented due to the complex directory structure that is idiosyncratic to each data set.

This thread is meant to discuss whether the IBE module should encapsulate IRSA's SIA and TAP interfaces, or whether those should be separate.

Start with the astroquery IBE module and its query_region_async URL construction, then read the linked IRSA IBE and TAP documentation. Determine whether IBE should encapsulate the SIA and TAP interfaces or use separate wrappers, and verify the chosen scope against the stated limitation that image fetching is not implemented.

Written by the indexing model from the issue text.