Add GCS and Azure Blob storage support to data export CLI documentation

[Copilot]

What's Changed in this PR

Documents new cloud storage backend options for the greptime cli data export command following the unified storage configuration refactoring in GreptimeTeam/greptimedb#7280.

Added GCS (Google Cloud Storage) options:

• --gcs, --gcs-bucket, --gcs-root (required)
• --gcs-scope, --gcs-credential-path, --gcs-credential, --gcs-endpoint (optional)

Added Azure Blob Storage options:

• --azblob, --azblob-container, --azblob-root, --azblob-account-name (required)
• --azblob-account-key, --azblob-endpoint, --azblob-sas-token (optional)

Updated --output-dir and --ddl-local-dir descriptions to reference new storage backends alongside existing S3/OSS support.

Fixed typo: "databasses" → "databases"

Checklist

• Please confirm that all corresponding versions of the documents have been revised.
• Please ensure that the content in sidebars.ts matches the current document structure when you changed the document structure.
• This change requires follow-up update in localized docs.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Update docs for refactor(cli): unify storage configuration for export command</issue_title>
<issue_description>A document change request is generated from GreptimeTeam/greptimedb#7280</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Update docs for refactor(cli): unify storage configuration for export command #2248

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