For software, documentation exists to facilitate its use - if the software doesn’t exist (or never will), there’s no need for documentation:
[…] perhaps it could be setup at a subdomain such as docs.haskell.org […]
code.haskell.org makes more sense - then the docs can be (rightly) paired with the corresponding software project. It could even be made a requirement for adding new projects - no useful docs, no new code! (Obviously, if an existing site e.g. hackage.haskell.org could be configured to operate in this manner, all the better…)
Then www.haskell.org only has to host one form of documentation - how to use Haskell. This could also be made conditional - each new proposal for a language extension must be accompanied by useful documentation, otherwise it will simply be ignored.
So, in this bright, shiny alternate reality:
you want to add your project ? Make sure its documentation is fit for purpose!
you want your project to stay? Make sure its documentation stays fit for purpose!
you found a problem with some piece of documentation for a project on code.haskell.org? Send that project some (polite!) feedback.
you are interested in improving some piece of documentation for a project on code.haskell.org? Join that project!
you want to add your proposal to extend the language? Make sure it’s adequately-documented, and make some time to answer any questions regarding your proposal.
you want your proposal to stay? Make sure it stays adequately-documented, and answer those questions.
you have a question about some language proposal? Ask it’s authors.
you want to improve a proposal’s documentation or answer a pertinent question? Join its authors.
…now back to this reality: documentation doesn’t appear at the flick of a wand - someone has to make the effort to write it. If it’s the project’s coders that are doing it, that’s less time for them to add that cool new feature or hunt down those annoying bugs - do try to remember that when the projects you like are running late with their next versions: coding (or documenting) will probably help more than complaining.