Docs: Added an FAQ (#7694)

### What problem does this PR solve?

### Type of change

- [x] Documentation Update

docs/faq.mdx

@@ -85,6 +85,14 @@ Where:
 
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+### Why not use other open-source vector databases as the document engine?
+
+Currently, only Elasticsearch and [Infinity](https://github.com/infiniflow/infinity) meet the hybrid search requirements of RAGFlow. Most open-source vector databases have limited support for full-text search, and sparse embedding is not an alternative to full-text search. Additionally, these vector databases lack critical features essential to RAGFlow, such as phrase search and advanced ranking capabilities.
+
+These limitations led us to develop [Infinity](https://github.com/infiniflow/infinity), the AI-native database, from the ground up.
+
+---
+
 ### Differences between demo.ragflow.io and a locally deployed open-source RAGFlow service?
 
 demo.ragflow.io demonstrates the capabilities of RAGFlow Enterprise. Its DeepDoc models are pre-trained using proprietary data and it offers much more sophisticated team permission controls. Essentially, demo.ragflow.io serves as a preview of RAGFlow's forthcoming SaaS (Software as a Service) offering.