Document that wsgiref is not meant for production

[sethmlarson]

Documentation

wsgiref is a reference implementation and not meant for production. However, there is no explicit security disclaimer on the documentation page. Let's explicitly add this so we can safely point to it if we get future security reports.

Linked PRs

• gh-144484: Warn users not to use wsgiref in production #144487
• [3.14] gh-144484: Warn users not to use wsgiref in production #144511
• [3.13] gh-144484: Warn users not to use wsgiref in production #144512
• [3.11] gh-144484: Warn users not to use wsgiref in production #144521
• [3.10] gh-144484: Warn users not to use wsgiref in production #144522
• [3.12] gh-144484: Warn users not to use wsgiref in production #144523

[benediktjohannes]

LGTM