Following the "if you do not have root access" installation docs for kubectl results in a "permission denied" error, so this change adds the appropriate chmod command to minimise confusion.
* Remove Google SDK instructions.
Ref: https://github.com/kubernetes/website/issues/20232
* Remove Google SDK instructions for MacOS
* Remove Google SDK instructions for Windows
* Delete install-kubectl-gcloud.md
The use of `apt-key` to install has also been removed as it is now deprecated
and will be last available in Debian 11 and Ubuntu 22.04.
Also updates the Docker repository setup instructions in container-runtimes.md,
to now refer to the respective instructions at https://docs.docker.com/engine/install/
which has already made the move to use the signed-by option.