Add concept page about the Kubernetes API client libraries explaining what they
do and list officially-supported & community-maintained client libraries.
The list is obtained from https://github.com/kubernetes/community/blob/master/contributors/devel/client-libraries.md
and I am planning to add a notice there urging people to update the
/docs/reference/client-libraries.md when they add something there.
Alternatively we could remove that page altogether.
Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>