redirect not working for updated contribute page

[tacaswell]

There should be a re-direct there:

matplotlib/doc/devel/contribute.rst

Lines 1 to 3 in 478dc5a

	.. redirect-from:: /devel/contributing
	.. _contributing:

We should merge this to avoid the redirect in our links, but we should also sort out why the redirect is not working.

Originally posted by @tacaswell in #27910 (comment)

[AnsonTran]

The issue seems to be with this line of code here: https://github.com/matplotlib/matplotlib/blob/5a4bb71f640a3c4dd8aac1af8952fe336da428b8/doc/sphinxext/redirect_from.py#L110C1-L110C5.

app.env.get_domain('redirect_from').redirects is an empty dictionary, so none of the redirect HTML pages get created. My guess is that it is being cleared somewhere?