DOC: Move indexslice below multiindex (#54144)

Author: coco90417

doc/source/reference/indexing.rst

@@ -251,10 +251,6 @@ IntervalIndex components
 
 MultiIndex
 ----------
-.. autosummary::
-   :toctree: api/
-
-   IndexSlice
 
 .. autosummary::
    :toctree: api/
@@ -314,6 +310,11 @@ MultiIndex selecting
    MultiIndex.get_indexer
    MultiIndex.get_level_values
 
+.. autosummary::
+   :toctree: api/
+
+   IndexSlice
+
 .. _api.datetimeindex:
 
 DatetimeIndex