DOC: Python as glue

This is the first part of some work to address #5239 and #4492. It removes mention of some dead Python/C interop projects, and perhaps more controversially, of `scipy.weave`. The latter project has been end-of-lifed by SciPy.

More work is coming up to address #4492 (Cython instead of Pyrex) specifically, but you may already merge this branch if you think it's a net improvement.
Maybe just an 'Other' section with links for pyinstant and weave.