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)

docs/references/python_api_reference.md

@@ -100,7 +100,6 @@ RAGFlow.create_dataset(
     embedding_model: Optional[str] = "BAAI/bge-large-zh-v1.5@BAAI",
     permission: str = "me", 
     chunk_method: str = "naive",
-    pagerank: int = 0,
     parser_config: DataSet.ParserConfig = None
 ) -> DataSet
 ```
@@ -148,10 +147,6 @@ The chunking method of the dataset to create. Available options:
 - `"one"`: One
 - `"email"`: Email
 
-##### pagerank, `int`
-
-The pagerank of the dataset to create. Defaults to `0`.
-
 ##### parser_config
 
 The parser configuration of the dataset. A `ParserConfig` object's attributes vary based on the selected `chunk_method`: