Allow setting local-doc-root in stack.yaml

Similarly to how we can specify [`local-bin-path`](https://docs.haskellstack.org/en/stable/yaml_configuration/#local-bin-path), it would be great to also be able to customize `local-doc-root` to make the Haddock output always be generated in a predictable place. Right now I think I'm going to have to add a line to my `Makefile` to copy the output from `$(stack path --local-doc-root)` to where it needs to be. This is not terrible, I suppose, but doesn't seem like it should be necessary.