Doc: add DataFrame.index.levels
- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [ ] [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
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] 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\user_guide\indexing.rst` file  adding a new feature.

> solved problem

BUG: MultiIndex.levels[x] of sliced DataFrame returns values from the non-sliced index #55148
[https://github.com/pandas-dev/pandas/issues/55148](url)

> Description:

Adding DataFrame.index.levels to doc will show the original value after reducing the data frame reason: pandas only updates the code at slicing time, not the level.

> Add content details

![屏幕截图 2023-10-07 125621(1)](https://github.com/pandas-dev/pandas/assets/143710553/6088db73-5573-4e38-b44f-d69273d61142)

@shiersansi this is the issue: #55435

I'm going to close this PR since I think it'll be easier for you to start from scratch, as everything in this PR needs to be reverted. But you're surely welcome to continue the work on this. Thank you!