[FIX] Clean up sphinx config + remove old wip example scripts

[liuzhenqi77]

No description provided.

[VinceBaz]

Some notes on this PR:

• It fixes some issues discussed in Issue [BUG] module 'netneurotools.stats' has no attribute 'gen_spinsamples' #166. More specifically, it removes the work in progress (wip) example files that should not have been generated by sphinx.

• It adds a _chk2_asarray function to replace the scipy function that was recently removed.

• sphinx_design was commented out because there was an issue with sphinx_design that made it uncompatible with Sphinx 9. It seems like this issue was fixed yesterday: ⬆️ Add Sphinx 9 support executablebooks/sphinx-design#255. We'll however keep it commented out for now, since it seems like sphinx_design might not even be necessary for our current documentation.