📖 Redirect on curl k8s stable version and kustomize install

[knabben]

What this PR does / why we need it:
Fix Kustomize bash installation documentation and redirect stable.txt download

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

[sbueringer]

Thx!
/lgtm

[fabriziopandini]

Wondering if we should remove this and instead refer to hack/ensure-kustomize.sh that installs the exact version of kustomize we rely on under the /hack/tools/bin folder 🤔

[sbueringer]

Wondering if we should remove this and instead refer to hack/ensure-kustomize.sh that installs the exact version of kustomize we rely on under the /hack/tools/bin folder 🤔

Is an alternative, but might be good if users don't have to clone the repo.

[fabriziopandini]

/lgtm

[vincepri]

/approve

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