Fix: Move pagerank field from create to update dataset API (#8217)

### What problem does this PR solve?

- Remove pagerank from CreateDatasetReq and add to UpdateDatasetReq
- Add pagerank update logic in dataset update endpoint
- Update API documentation to reflect changes
- Modify related test cases and SDK references

#8208

This change makes pagerank a mutable property that can only be set after
dataset creation, and only when using elasticsearch as the doc engine.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

sdk/python/ragflow_sdk/ragflow.py

@@ -56,7 +56,6 @@ class RAGFlow:
         embedding_model: Optional[str] = "BAAI/bge-large-zh-v1.5@BAAI",
         permission: str = "me",
         chunk_method: str = "naive",
-        pagerank: int = 0,
         parser_config: Optional[DataSet.ParserConfig] = None,
     ) -> DataSet:
         payload = {
@@ -66,7 +65,6 @@ class RAGFlow:
             "embedding_model": embedding_model,
             "permission": permission,
             "chunk_method": chunk_method,
-            "pagerank": pagerank,
         }
         if parser_config is not None:
             payload["parser_config"] = parser_config.to_json()