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.

@pijucha looks really good. just a couple of comments.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13298?src=pr) is **84.23%**

> Merging [#13298](https://codecov.io/gh/pydata/pandas/pull/13298?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13298   diff @@
==========================================
  Files           138        138          
  Lines         50681      50681          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42682      42687     +5   
+ Misses         7999       7994     -5   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [9e7bfdd...39c0676](https://codecov.io/gh/pydata/pandas/compare/9e7bfdd5996d90c774cf6ebd39c6c0779469e545...39c067629998f79184b87330a2ba7ba8ee861782)
 @pijucha looks good. just some more validation tests and some doc changes.
 @jreback  I've made the changes. (Tests still pending.)
 thanks @pijucha nice PR! I slightly edited the docs so take a look when built.
 @jreback Thanks
