doc: remove WSL install instructions and point to upstream
doc: remove WSL install instructions and point to upstream

There's not really any need to us to have to replicate (ever-changing)
instructions for installing an operating system in our build
documentation.

caraka/gridcoinresearch,caraka/gridcoinresearch,caraka/gridcoinresearch,caraka/gridcoinresearch,caraka/gridcoinresearch,caraka/gridcoinresearch