DOC: add section on groupby().rolling/expanding/resample
 - [x] closes #14759 

This is my first PR here. Not sure about the missing entries in API reference. Need some guidance on that. The same doubt about the best place to put the xref in computation.rst and timeseries.rst


Thanks.


@jorisvandenbossche can you help me with more information about the failed test below?

 @wandersoncferreira you can ignore the AppVeyor error, that is a problem on our end unrelated to this PR. Travis did pass. ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14801?src=pr) is 85.28% (diff: 100%)
> Merging [#14801](https://codecov.io/gh/pandas-dev/pandas/pull/14801?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #14801   diff @@
==========================================
  Files           144        144          
  Lines         50947      50968    +21   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43445      43467    +22   
+ Misses         7502       7501     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [5d0e157...b0fa254](https://codecov.io/gh/pandas-dev/pandas/compare/5d0e15716c5466e3a12691fb6c257734396ca451...b0fa25439669ad801ccc33b7035c80fc1f2b3ba8?src=pr) @jreback thanks for your comments. Let me know if I can make it even better. @wandersoncferreira Thanks!