x/website: source install instructions missing new windows ports

[mcandre]

Go 1.17 adds support of 64-bit ARM architecture on Windows (the windows/arm64 port). This port supports cgo.

https://golang.org/doc/go1.17#windows

However, this other Go documentation omits the arm and arm64 architecture combinations for Windows:

https://golang.org/doc/install/source#environment

[gopherbot]

Change https://golang.org/cl/343216 mentions this issue: _content/doc/install: add windows/arm and windows/arm64

[mknyszek]

I didn't ask to deploy this to the website since there are other things to fix as well. But rest assured it'll update whenever the next deploy happens.