[docs](iceberg) Document REST catalog labels since 5.0.0 - #4532

Document the Iceberg REST Catalog $labels system table introduced by apache/doris#69135, available since Doris 5.0.0.

Update the English and Chinese current-version Iceberg Catalog guides with the field definitions, query examples and expected results, cache refresh instructions, and restrictions. Cover REST-only availability, nested fields, schema changes, empty results, snapshot-independent labels, and the server-side scan planning restriction. Add support-matrix and system-table version notes.

Validation

• Docusaurus MDX compilation, front matter, and section-link targets passed for both pages.
• Internal-link checks passed for both pages; existing external links are report-only.
• All five SQL and expected-output blocks match between English and Chinese.
• git diff --check passed.
• SEO checking reported existing duplicate-title warnings across versions and locales; front matter is unchanged.
• AI self-review found no additional issues. Page paths, navigation, and site code are unchanged. Older versions are not updated because this feature starts in 5.0.0.

Versions

• dev
• 4.x
• 3.x
• 2.1 or older (not covered by version/language sync gate)

Languages

• Chinese
• English

Docs Checklist

• Checked by AI
• Test Cases Built (documentation only; no runtime test cases added or run)
• Updated required version and language counterparts, or explained why not
• If only one language changed, confirmed whether source/translation counterparts need sync (not applicable: both languages updated)