DOC: Add missing attributes to Styler class
- [x] closes #60815
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Validated changes using `python scripts/validate_docstrings.py pandas.io.formats.style.Styler`

### Tasks performed

- Added documentation for `index` and `columns` under `Attributes` in `pandas.io.formats.style.Styler` as [discussed](https://github.com/pandas-dev/pandas/issues/60815#issuecomment-2665924948).

### Additional Comments

- First PR on this project, your feedback is highly valuable to me, looking forward to contributing more and more in the coming days!

Thanks @Rishab260 