Making scipy.org just about SciPy the library

[rgommers]

The top-level "SciPy ecosystem" page has gotten way too limiting, and SciPy the library is underserved. We should/will make scipy.org just about SciPy the library (perhaps similar in scope to numpy.org). We had a thread about this on the mailing list last September, and people seem mostly in agreement, so opening an issue to capture that here: https://mail.python.org/pipermail/scipy-dev/2019-September/023731.html