Update thumbnail-spec.sgml (clone of !42 to fix CI trouble)

This is an unmodified copy of !42 (closed), because that got stuck on CI formalities and (as far as I can judge) is otherwise ready.

I'll use this branch and PR to see whether the CI problem discussion around !42 (comment 2044723) can be resolved by me who is a user without any special privileges. (Might be; earlier tests of just pushing the branch erred out after "Checking if the job is part of an official MR pipeline...").