Where does stack haddock (or stack build --haddock) place the documentation that it generates?
That depends on where the package that the haddocks are generated for "belongs". Haddocks for "local" packages, that are part of a stack project, will be placed inside the .stack-work directory inside the project directory. Haddocks for snapshot packages will be placed in the stack root directory, typically ~/.stack.
The easiest way to discover the exact path is stack haddock --open. For example, run stack haddock --open base or stack haddock --open my-pkg in a project that contains a package with that name.