WTF8 reference in documentation redirects

[trypsynth]

Currently, whenever the docs talk about WTF8, they link to https://simonsapin.github.io/wtf-8/. However, that now redirects to https://wtf-8.codeberg.page/. Visiting the original ink redirects you automatically after 10 seconds, but the docs should probably point to the real URL.

[squeek502]

Fixed by #25532