ENH: add kw `levels` to `Styler.hide_index` to hide only specific levels
- [x] closes #25475
- [x] tests added / passed
- [x] whatsnew entry 1.4.0

![Screen Shot 2021-07-21 at 14 13 35](https://user-images.githubusercontent.com/24256554/126486650-af383d0b-3372-4c19-bb4a-db19dff25e6f.png)

@jreback pinging on green failures unrelated