doc: Add MSYS2 to the Windows setup guide

Since MSYS2 is far more stable, reliable, maintained and easy
to set up than MinGW as a Windows development environment,
default to it for the official Windows Getting Started guide,
including instructions to compile for both x86 and ARM with it.

Change-Id: Iaba61498b3b16a8ee1edbff953de16c275282c7b
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>