[DOCS] Chinese translation of Box2D SQL documentation

[jiayuasu]

Follow-up to #2966 (initial Box2D SQL docs) and #2969 (ST_DWithin(Box2D, Box2D, distance) overload). Tracks the Chinese translation of the new Box2D documentation surface.

Why this is its own issue

The i18n epic #2867 closed Phase 8 (Geometry / Geography function reference) without translating per-function SQL pages — those rely on mkdocs-static-i18n's fallback_to_default: true to show English content under /zh/. The Box2D surface follows the same convention today: the nav label Box2D Functions is translated centrally via nav_translations (added in #2966), and the per-function pages render English.

This issue tracks if/when we want to break that convention for Box2D and ship a fuller /zh/ Box2D reference.

Scope

If we decide to translate Box2D pages, the deliverables are:

• docs/api/sql/box2d/Box2D-Functions.zh.md — master page (intro, semantic notes, constructor / accessor / predicate / function tables, aggregates, type conversion, query optimization).
• docs/api/sql/box2d/Box2D-Constructors/ST_Box2D.zh.md
• docs/api/sql/box2d/Box2D-Constructors/ST_MakeBox2D.zh.md
• docs/api/sql/box2d/Box2D-Constructors/ST_GeomFromBox2D.zh.md
• docs/api/sql/box2d/Box2D-Accessors/ST_XMin.zh.md
• docs/api/sql/box2d/Box2D-Accessors/ST_YMin.zh.md
• docs/api/sql/box2d/Box2D-Accessors/ST_XMax.zh.md
• docs/api/sql/box2d/Box2D-Accessors/ST_YMax.zh.md
• docs/api/sql/box2d/Box2D-Predicates/ST_BoxIntersects.zh.md
• docs/api/sql/box2d/Box2D-Predicates/ST_BoxContains.zh.md
• docs/api/sql/box2d/Box2D-Predicates/ST_DWithin.zh.md
• docs/api/sql/box2d/Box2D-Functions/ST_Expand.zh.md
• docs/api/sql/box2d/Box2D-Functions/ST_AsText.zh.md
• docs/api/sql/Aggregate-Functions/ST_Extent.zh.md
• The two new Optimizer.md Box2D sections (Box2D filter pushdown, Box2D spatial join) need anchors that resolve under /zh/Optimizer/. Cross-links to those anchors from the translated Box2D pages should work via the i18n plugin's fallback.

Notes

• Translation conventions established by Phase 8 PRs apply: function signatures, parameter names, code samples, and SQL output stay English; descriptive prose, parameter descriptions, and Description / Format / Example headings are translated.
• Recommend deferring until there is appetite to do the same for Geometry-Functions.zh.md and the other per-function pages, so we don't create an asymmetric situation where Box2D is the only translated SQL function reference. Alternatively, do Box2D first as a pilot to validate the per-function translation convention.

Out of scope

• The English Box2D docs themselves (already landed in [DOCS] Add Box2D SQL documentation #2966 / [GH-2971] Add ST_DWithin(Box2D, Box2D, distance) overload #2969).
• Translations of other SQL reference pages — those should be filed as their own follow-ups under Add Chinese version of the documentation #2867.