add gcp-pubsub doc

docs/messaging/gcp-pubsub.md

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+<!--- Hugo front matter used to generate the website version of this page:
+linkTitle: gcpPubsub
+--->
+
+# Semantic Conventions for Google Cloud Pub/Sub
+
+**Status**: [Experimental][DocumentStatus]
+
+The Semantic Conventions for [Google Cloud Pub/Sub](https://cloud.google.com/pubsub) extend and override the [Messaging Semantic Conventions](README.md) that describe common messaging operations attributes in addition to the Semantic Conventions described on this page.
+
+`messaging.system` MUST be set to `"gcp_pubsub"`.
+
+[DocumentStatus]: https://github.com/open-telemetry/opentelemetry-specification/tree/v1.26.0/specification/document-status.md