docs: Add openGauss vector store documentation (#30742)

Hey LangChain community! 👋 Excited to propose official documentation for
our new openGauss integration that brings powerful vector capabilities
to the stack!

### What's Inside 📦  
1. **Full Integration Guide**  
Introducing
[langchain-opengauss](https://pypi.org/project/langchain-opengauss/) on
PyPI - your new toolkit for:
   🔍 Native hybrid search (vectors + metadata)  
   🚀 Production-grade connection pooling  
   🧩 Automatic schema management  

2. **Rigorous Testing Passed** ✅  
![Benchmark
Results](https://github.com/user-attachments/assets/ae3b21f7-aeea-4ae7-a142-f2aec57936a0)
   - 100% non-async test coverage  

ps: Current implementation resides in my personal repository:
https://github.com/mpb159753/langchain-opengauss, How can I transfer
process to langchain-ai org?? *Keen to hear your thoughts and make this
integration shine!* ✨

---------

Co-authored-by: Eugene Yurtsev <eugene@langchain.dev>
Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

docs/scripts/vectorstore_feat_table.py

@@ -140,6 +140,17 @@ def get_vectorstore_table():
             "Local/Cloud": "Local",
             "IDs in add Documents": True,
         },
+        "openGauss": {
+            "Delete by ID": True,
+            "Filtering": True,
+            "similarity_search_by_vector": True,
+            "similarity_search_with_score": True,
+            "asearch": False,
+            "Passes Standard Tests": True,
+            "Multi Tenancy": False,
+            "Local/Cloud": "Local",
+            "IDs in add Documents": True,
+        },
         "QdrantVectorStore": {
             "Delete by ID": True,
             "Filtering": True,