[Documentation] Fix Python documentation CSS (#33908)

Fix python documentation website sidebar.

### Before:
<img width="1168" alt="Screenshot 2023-07-27 at 4 49 25 PM"
src="https://github.com/grpc/grpc/assets/24593237/1434ac63-7fc3-4f5c-a931-9a939fb67de6">

### After:
![Screenshot 2023-07-28 at 1 04 01
PM](https://github.com/grpc/grpc/assets/24593237/c7cc16b6-926f-4c8b-bc54-6cedfebb62b4)


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doc/python/sphinx/_static/custom.css

@@ -1,3 +1,8 @@
 dl.field-list > dt {
     word-break: keep-all !important;
 }
+.sphinxsidebar {
+    overflow-y: scroll;
+    top: 0;
+    bottom: 0;
+}