BUG: Fix describe(): percentiles (#13104), col index (#13288)
- [x] closes #13104, #13288
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

BUG #13104:
- Percentiles are now rounded to the least precision that keeps
  them unique.
- Supplying duplicates in percentiles will raise ValueError.

BUG #13288
- Fixed a column index of the output data frame.
  Previously, if a data frame had a column index of object type and
  the index contained numeric values, the output column index could
  be corrupt. It led to ValueError if the output was displayed.
