gcpdiag: add page (#9678)

* Create gcpdiag.md

* Rename gcpdiag.md to pages/common/gcpdiag.md

* Update gcpdiag.md

* Update pages/common/gcpdiag.md

* Update pages/common/gcpdiag.md

* Update pages/common/gcpdiag.md

* Update pages/common/gcpdiag.md

* Update pages/common/gcpdiag.md

* Update pages/common/gcpdiag.md

* Update pages/common/gcpdiag.md

* Update pages/common/gcpdiag.md

* Update pages/common/gcpdiag.md

pages/common/gcpdiag.md

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+# gcpdiag
+
+> Google Cloud Platform troubleshooting and diagnostics tool.
+> Run in a docker container or in GCP Cloudshell.
+> More information: <https://github.com/GoogleCloudPlatform/gcpdiag>.
+
+- Run `gcpdiag` on your project, returning all rules:
+
+`gcpdiag lint --project={{gcp_project_id}}`
+
+- Hide rules that are ok:
+
+`gcpdiag lint --project={{gcp_project_id}} --hide-ok`
+
+- Authenticate using a service account private key file:
+
+`gcpdiag lint --project={{gcp_project_id}} --auth-key {{path/to/private_key}}`
+
+- Search logs and metrics from a number of days back (default: 3 days):
+
+`gcpdiag lint --project={{gcp_project_id}} --within-days {{number}}`
+
+- Display help:
+
+`gcpdiag lint --help`