Refactor docs of RAGFlow admin (#12361)

### What problem does this PR solve?

as title

### Type of change

- [x] Documentation Update

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

docs/guides/admin/_category_.json

@@ -0,0 +1,8 @@
+{
+  "label": "Administration",
+  "position": 6,
+  "link": {
+    "type": "generated-index",
+    "description": "RAGFlow administration"
+  }
+}