docs: improve ai-prompt-guard plugin docs

[Yilialinn]

Description

Improve the ai-prompt-guard plugin doc and update the Chinese version accordingly.

Which issue(s) this PR fixes:

Fixes #

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[Copilot]

Pull request overview

This PR refreshes the ai-prompt-guard plugin documentation (English + Chinese) to better explain allow/deny behavior and to provide fuller, multi-environment configuration examples for using it alongside ai-proxy.

Changes:

• Expanded the plugin description and clarified allow/deny evaluation behavior.
• Added canonical link metadata and significantly expanded example sections (Admin API, ADC, and Ingress Controller variants).
• Updated the Chinese documentation to match the English structure and content.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 16 comments.

File	Description
docs/en/latest/plugins/ai-prompt-guard.md	Adds canonical metadata, rewrites description/attributes, and adds detailed usage examples across deployment methods.
docs/zh/latest/plugins/ai-prompt-guard.md	Mirrors the English doc improvements and adds the same expanded examples and clarifications in Chinese.

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