Add Stable: No status field to merge_imports configuration docs (#6828)

* docs: Add `Stable: No` status field and tracking issue link for `merge_imports`
---------

Co-authored-by: zzcn <zzcn.leo@qq.com>

Author: zichen0116

Configurations.md

@@ -2059,6 +2059,7 @@ This option is deprecated. Use `imports_granularity = "Crate"` instead.
 
 - **Default value**: `false`
 - **Possible values**: `true`, `false`
+- **Stable**: No (tracking issue: [#3362](https://github.com/rust-lang/rustfmt/issues/3362))
 
 #### `false` (default):