[Doc]Add Chinese translation for documentation (#1870)

### What this PR does / why we need it?

This PR adds a complete Chinese translation for the documentation using
PO files and the gettext toolchain. The goal is to make the
documentation more accessible to Chinese-speaking users and help the
community grow.

### Does this PR introduce any user-facing change?

Yes. This PR introduces Chinese documentation, which users can access
alongside the original English documentation. No changes to the core
code or APIs.

### How was this patch tested?

The translated documentation was built locally using the standard
documentation build process (`make html` or `sphinx-build`). I checked
the generated HTML pages to ensure the Chinese content displays
correctly and matches the original structure. No code changes were made,
so no additional code tests are required.

vLLM version: v0.9.2  
vLLM main: vllm-project/vllm@5780121

---

Please review the translation and let me know if any improvements are
needed. I am happy to update the translation based on feedback.

- vLLM version: v0.9.2
- vLLM main:
vllm-project/vllm@7ba34b1

---------

Signed-off-by: aidoczh <aidoczh@163.com>
Signed-off-by: Yikun Jiang <yikunkero@gmail.com>
Co-authored-by: Yikun Jiang <yikunkero@gmail.com>

Author: aidoczh

docs/Makefile

@@ -19,3 +19,7 @@ help:
 %: Makefile
 	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
 
+intl:
+	sphinx-intl build
+	@$(SPHINXBUILD) -b html -D language=zh_CN "$(SOURCEDIR)" "$(BUILDDIR)/html/zh-cn" $(SPHINXOPTS) $(O)
+

docs/README.md

@@ -11,13 +11,14 @@ pip install -r requirements-docs.txt
 # Build the docs.
 make clean
 make html
-```
 
-## Open the docs with your browser
+# Build the docs with translation
+make intl
 
-```bash
+# Open the docs with your browser
 python -m http.server -d _build/html/
 ```
 
-Launch your browser and open http://localhost:8000/.
-
+Launch your browser and open:
+- English version: http://localhost:8000
+- Chinese version: http://localhost:8000/zh_CN

docs/requirements-docs.txt

@@ -7,3 +7,4 @@ sphinx-togglebutton
 myst-parser
 msgspec
 sphinx-substitution-extensions
+sphinx-intl

docs/source/conf.py

@@ -88,8 +88,8 @@
 #
 # This is also used if you do content translation via gettext catalogs.
 # Usually you set "language" from the command line for these cases.
-language = 'en'
-
+locale_dirs = ['locale/']   
+gettext_compact = False   
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.
 # This pattern also affects html_static_path and html_extra_path.