ENH: Support na_position for sort_index and sortlevel
- [x] closes #51612 (Replace xxxx with the GitHub issue number)
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

I think the special casing in sort level was done for performance reason. Avoiding the use of categorical solves this. Both branches were equally fast 

```
%timeit midx.sortlevel(level=[0, 1], ascending=[True, True])
58.8 ms ± 674 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
%timeit midx.sortlevel(level=[0, 1], ascending=True)
60.7 ms ± 278 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
```
Thanks @phofl 