Upstream tpgb 3238: docs for cluster notification_config

[melinath]

Upstreams hashicorp/terraform-provider-google-beta#3238.

Third-party only, no CLA required.

Guide docs: https://cloud.google.com/kubernetes-engine/docs/how-to/cluster-upgrade-notifications
API docs: https://cloud.google.com/kubernetes-engine/docs/reference/rest/v1/projects.locations.clusters

Release Note Template for Downstream PRs (will be copied)

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[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 1 file changed, 21 insertions(+))
Terraform Beta: Diff ( 2 files changed, 22 insertions(+), 1 deletion(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=187377"