Feat: Markdown add image (#7124)

### What problem does this PR solve?

https://github.com/infiniflow/ragflow/issues/6984

1. Markdown parser supports get pictures
2. For Native, when handling Markdown, it will handle images
3. improve merge and 

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

---------

Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

web/src/interfaces/database/knowledge.ts

@@ -103,7 +103,7 @@ export interface IChunk {
   content_with_weight: string;
   doc_id: string;
   doc_name: string;
-  img_id: string;
+  image_id: string;
   important_kwd?: string[];
   question_kwd?: string[]; // keywords
   tag_kwd?: string[];