Remove the visibilty of RAGFlow API (#1196)

### What problem does this PR solve?

_Briefly describe what this PR aims to solve. Include background context
that will help reviewers understand the purpose of the PR._

### Type of change

- [x] Documentation Update

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

docs/references/ragflow_api.md

@@ -1,7 +1,3 @@
----
-sidebar_position: 1
-slug: /api
----
 
 # API reference