docs: add gc_table and gc_regions admin functions

[Copilot]

Documents two new admin functions introduced in greptimedb#7619 for manually triggering garbage collection of orphaned SST files in object storage.

Changes

• docs/reference/sql/admin.md — Added gc_table and gc_regions to the function list and examples; updated front matter keywords/description
• i18n/zh/.../reference/sql/admin.md — Chinese translation of the same additions

New functions

-- GC orphaned SST files for a dropped table
admin gc_table("test");
admin gc_table("test", true);  -- full file listing mode

-- GC orphaned SST files for specific regions
admin gc_regions(1, 2, 3);
admin gc_regions(1, 2, 3, true);  -- full file listing mode

Both functions return the number of processed regions. The optional full_file_listing boolean (default false) forces a thorough scan of all files in object storage.

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.

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[fengjiachun]

LGTM