Thanks to the kind help of @mpilgrem, errors.haskell.org now supports the latest release candidate of Stack in addition to the development branch of GHC. If you’re looking for a way to contribute to Haskell tools that can be done in small bits of free time and only requires writing a bit of Markdown, please consider helping out with the documentation here!
So far, I documented one of Stack’s error codes, which you can use as a starting point if you’d like to contribute.
If you’re interested in helping out, CONTRIBUTING.md in the site repository has instructions.