Update KubeOne docs for ARM based installation

Installing KubeOne from an ARM based System is NOT mentioned in the docs.

All code examples have amd64 "hard coded". Update would be great.

https://docs.kubermatic.com/kubeone/v1.11/getting-kubeone/

This worked fine for me:

OS=$(uname)
VERSION=$(curl -w '%{url_effective}' -I -L -s -S https://github.com/kubermatic/kubeone/releases/latest -o /dev/null | sed -e 's|.*/v||')

curl -LO "https://github.com/kubermatic/kubeone/releases/download/v${VERSION}/kubeone_${VERSION}_${OS}_arm64.zip"

unzip kubeone_${VERSION}_${OS}_arm64.zip -d kubeone_${VERSION}_${OS}_arm64
sudo mv kubeone_${VERSION}_${OS}_arm64/kubeone /usr/local/bin

Start with the KubeOne v1.11 getting-started page linked in the issue and review its installation examples for hard-coded amd64 references. Update the examples to cover ARM-based installation using the provided arm64 download commands while retaining the existing amd64 path. Done means the rendered documentation clearly explains both architectures.

Written by the indexing model from the issue text.