ci: build haddock/sphinx-html for wasm jobs

This commit enables building haddock & sphinx-html documentation for
wasm targets. The docs are useful for end users and should be tested
in CI.

I've omitted pdf & manpage generation for the wasm target; I've never
found the pdf version of docs to be useful, and the manpage only
contains `ghc.1` where we really want `wasm32-wasi-ghc.1` but that
should be a separate issue to fix.