docs: add image mirror guide for v1.9.x

[jake11-oho]

Description

Add a User Guide documenting the rock image mirror command for mirroring custom Docker images to the ROCK ACR (rock-instances).

Content

• Registry regions: Singapore (default target) and Shanghai (synced via ACR replication)
• rock image mirror command reference with all parameters
• Usage examples: public/private images in local mode, remote mode
• Direct-to-Shanghai mirroring via --cluster vpc-nt-a to bypass ACR sync delays
• Image name mapping rules and build result format

Files

• docs/versioned_docs/version-1.9.x/User Guides/image_mirror.md (English)
• docs/i18n/zh-Hans/.../version-1.9.x/User Guides/image_mirror.md (Chinese)