doc: Improve windows build instructions using Linux subsystem

I did a build on a windows 10 laptop and took notes, and tried
to improve the document:

• It's the Linux subsystem for Windows, not the other way around.

• Split out dependencies: general ones, 64-bit, 32-bit. Remove the reference to build-unix.md, easy enough to be self-contained.

• Place 64-bit instructions first. 99% will want these.

• Installation instructions: recommend using / for prefix, same as we do on gitian builds. This will allow copying the files to a usable (from Windows) place using just make DESTDIR=....

• Remove double spaces / consistent width reformatting.