sudoku-backend API doc doesn't build

[nabijaczleweli]

And therefore doesn't upload so it's not available under the specified link.

This is due to rust-lang/rust#51661 and will be fixed automatically when that lands.

[nabijaczleweli]

After discussion and a diesel issue, this is the new tracking target.
(However, docs do build now, except I'm livepatching the diesel source on CI.)