doc: fix installation on windows instructions

Add more installation details for building zephyr on Windows
including what to do with the ISSM tar.gz file that's
downloaded, proxy settings if behind a firewall, adding tar
to the dependencies (needed to extract the ISSM tar.gz file),
linking to the downloads page for the ARM toolchain, and
fixing command lines to correctly use `make BOARD=`. Verified
instructions work on a Windows 10 system.

Change-Id: I04e5f8e46df7630868568b90388dc65bb9baa4c9
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>