feat(zh): add Chinese translations in zh/ directory

Add Chinese (Simplified) translations for all documentation, organized
under a dedicated zh/ directory that mirrors the English folder structure.

Co-authored-by: tanqingkuang <tanqingkuang@users.noreply.github.com>

Translations originally contributed by @tanqingkuang in #45.
Restructured from *-CN.md suffix pattern into zh/ directory to prevent
the EPUB builder (scripts/build_epub.py collect_folder_files) from
picking up Chinese files via glob("*.md") inside module folders.

scripts/check_cross_references.py

@@ -3,6 +3,7 @@
 
 import re
 import sys
+import unicodedata
 from pathlib import Path
 
 IGNORE_DIRS = {
@@ -63,7 +64,7 @@ def main() -> int:
     errors = []
 
     for file_path in iter_md_files():
-        content = file_path.read_text()
+        content = file_path.read_text(encoding="utf-8")
         # Strip code blocks before scanning for links/anchors to avoid false positives
         # from documentation examples inside code fences.
         scannable = strip_code_blocks(content)