Update MSVC instructions

[ChrisDenton]

This simplifies the Windows install instructions and points people towards the rustup docs for more help. The rustup installer itself should give instructions on installing Visual Studio and the rustup documentation can offer more in-depth, up to date, guidance than can be provided by The Book.

cc @rbtcollins, for linking to rustup docs (if this is merged, I'll also need to add a note to rustup's dev docs about keeping the link url stable)

[chriskrycho]

I love this suggested simplification, @ChrisDenton – but I definitely want to make sure we have everything lined up with rustup etc.! Who all (@rbtcollins?) do we need to make sure is on the same page here so everything works even more nicely for Windows users?

[rbtcollins]

I think its fine to link to the rustup book, with the suggested note in the docs to keep the url stable.

[rami3l]

@chriskrycho We're already ensuring page URL stability in the mdBook docs published by Rustup. Plus, I've made rust-lang/rustup#3762 to explicitly state that in our dev guide, so I believe this PR is good to go.

[chriskrycho]

Outstanding – thank you all!