docs(trigger): for and keep_firing_for

[fengys1996]

What's Changed in this PR

This pr mainly introduces describe of for and keep_firing_for clauses in trigger.

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.

[Copilot]

Pull request overview

This PR adds documentation for two new optional clauses in the GreptimeDB Trigger syntax: FOR and KEEP_FIRING_FOR. These clauses provide control over alert state transitions, similar to Prometheus Alerting Rules. The documentation updates both the English and Chinese localized versions.

• Added FOR clause documentation to control the pending duration before an alert fires
• Added KEEP_FIRING_FOR clause documentation to control minimum firing duration after an alert triggers
• Updated query expression descriptions to clarify alert instance concepts

Reviewed changes

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

File	Description
docs/reference/sql/trigger-syntax.md	Added English documentation for FOR and KEEP_FIRING_FOR clauses in trigger syntax, clarified alert instance behavior and terminology
i18n/zh/docusaurus-plugin-content-docs/current/reference/sql/trigger-syntax.md	Added Chinese documentation for FOR and KEEP_FIRING_FOR clauses in trigger syntax, clarified alert instance behavior and terminology

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