Broken link from working mode to Stages.md

[jyasskin]

Currently https://whatwg.org/working-mode#new-proposals contains a link to https://whatwg.org/Stages.md, but the right URL is https://whatwg.org/stages. It looks like this is supposed to get fixed in https://github.com/whatwg/whatwg.org/blob/main/convert_policy.py based on https://github.com/whatwg/sg/blob/main/policy-link-mapping.txt, but it's not obvious to me what's getting missed.

[cwilso]

PR: https://github.com/whatwg/sg/pull/229

[janbrasna]

The fix above has been released.