docs: document ocr completion command

[Mainul999-bot]

Fixes #639

Documents the ocr completion command (bash/zsh/fish/powershell), which is implemented but has zero documentation.

CLI Reference (en, zh, ja)

• New ## ocr completion section alongside the existing command sections
• Command format: ocr completion [bash|zsh|fish|powershell]
• Per-shell one-shot and persistent setup instructions, mirroring the help text in cmd/opencodereview/completion.go

Installation (en, zh, ja, ru)

• New "Enable shell completion" section after "Verifying the install"
• One-line explanation, quick bash/zsh/fish examples, and a link to the CLI Reference

Content matches the shells and setup steps in cmd/opencodereview/completion.go.

[CLAassistant]

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[lizhengfeng101]

@Mainul999-bot Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

[github-actions]

✅ OpenCodeReview: No supported files changed.

[lizhengfeng101]

Hi @Mainul999-bot, thanks for the contribution! Could you please sign the CLA so we can move forward with merging this PR?

Your PR is currently our first priority for this change. However, if the CLA remains unsigned for an extended period, we may need to merge an alternative PR to keep the project moving.

Please let us know if you have any questions about the CLA process.