DOC: Update python as glue doc.

[liang3zy22]

As @mattip suggested, to close #7035 .

[rossbar]

+1 for me from adding this warning. At first I was inclined to see if there was appetite to remove it entirely, but then there are sections on f2py which likely are still relevant for NumPy. Longer term I think it'd be worthwhile to refactor the article to focus largely on f2py and refer readers to external resource(s) for general info on extensions.

I suggest to make the note more prominent so it stands out at the top of the doc. I'll let @mattip weigh in whether there are other ways this can be improved; LGTM!

[mattip]

LGTM, thanks. Once CI runs, a reviewer should look at the rendered doc to make sure it works as expected before merging.

[rossbar]

Rendered docs look good, thanks @liang3zy22 and @mattip !