Adding Docs for Datasource google_gke_hub_membership_binding

[NA2047]

Fix for hashicorp/terraform-provider-google#21737

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docs: added docs for `google_gke_hub_membership_binding` data source

[github-actions]

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 1 file changed, 54 insertions(+))
google-beta provider: Diff ( 1 file changed, 54 insertions(+))

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 1 file changed, 54 insertions(+))
google-beta provider: Diff ( 1 file changed, 54 insertions(+))

[zli82016]

Thanks.