Move client-libraries.md to kubernetes.io
Move client-libraries.md to kubernetes.io

This content is now served at https://kubernetes.io/docs/reference/client-libraries/
and removing this page will encourage people to keep the website up to date.

Signed-off-by: Ahmet Alp Balkan <c786c8527fd6be7431bb94ca48b6f76cc06787cc@google.com>

kubernetes/sig-release