add everyting for docs

docs/snippets/modules/chains/document/combine_docs.mdx

@@ -0,0 +1,9 @@
+```python
+class BaseCombineDocumentsChain(Chain, ABC):
+    """Base interface for chains combining documents."""
+
+    @abstractmethod
+    def combine_docs(self, docs: List[Document], **kwargs: Any) -> Tuple[str, dict]:
+        """Combine documents into a single string."""
+
+```