DOC: Adding repository pandas_exercises to tutorials.rst
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14980?src=pr) is 84.66% (diff: 100%)
> Merging [#14980](https://codecov.io/gh/pandas-dev/pandas/pull/14980?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #14980   diff @@
==========================================
  Files           144        144          
  Lines         51047      51056     +9   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43216      43226    +10   
+ Misses         7831       7830     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [d7e8f31...6985079](https://codecov.io/gh/pandas-dev/pandas/compare/d7e8f316dc7155252fa47ea23a3074d7d0197e54...6985079f3c083e860e7cc938c07d00001a7e3fae?src=pr) @guipsamora Seems like a nice repo of exercises! 
BTW, if you are interested in a scriptable way to automatically create the versions with and without solutions, you are welcome to check out https://github.com/jorisvandenbossche/nbtutor