Add binary source to docs/obtaining.rst

[userdocs]

I have been building these fully static musl based for some time that cover most key architectures, require no dependencies and are not limited by glibc.

There are 100% Github Action based build and release binaries.

./iperf3-amd64 --version
iperf 3.13 (cJSON 1.7.15)
Optional features available: CPU affinity setting, IPv6 flow label, TCP congestion algorithm setting, sendfile / zerocopy, socket pacing, authentication, bind to device, support IPv4 don't fragment

https://github.com/userdocs/iperf3-static

https://github.com/userdocs/iperf3-static/releases/latest

Alpine Arch	Docker platform arch	Docker hub image
armhf	linux/arm/v6	arm32v6/alpine:edge
armv7	linux/arm/v7	arm32v7/alpine:edge
aarch64	linux/arm64	arm64v8/alpine:edge
ppc64le	linux/ppc64le	ppc64le/alpine:edge
s390x	linux/s390x	s390x/alpine:edge
x86	linux/i386	i386/alpine:edge
x86_64	linux/amd64	amd64/alpine:edge

I think they are a useful source for a binary location in:

https://github.com/esnet/iperf/blob/master/docs/obtaining.rst