[DOC] Document the usage of the plugin repository-gcs

[gaobinlong]

What do you want to do?
The core plugin repository-gcs exists in OpenSearch since 1.0.0, but we don't document the usage, and I see there're many questions about it in the forum: https://forum.opensearch.org/t/plugin-repository-gcs-not-working-for-snapshots/11126, we need to document the usage in our official documentation.

Tell us about your request. Provide a summary of the request.
repository-gcs is similar with repository-s3 and repository-azure, which are used to take and restore snapshot, we may add the documentation here: https://opensearch.org/docs/latest/tuning-your-cluster/availability-and-recovery/snapshots/snapshot-restore/.

Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
all

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.

[gaobinlong]

Need to ping the snapshot team to complete this work.

[Naarcha-AWS]

Thanks for submitting this @gaobinlong. I'll assign to you for now but let me know if I should update it.