DOC/DEPR: pivot_annual
- [x] closes #736
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Updated cookbook attaching SO link of original issue.

BTW, `pivot_annual` has a similar functionality but it looks to be replaced by `pivot_table` and datetime property access. Because the function is not on top `pd` namespace, shouldn't deprecate it?

yes this lgtm. @jorisvandenbossche any objections?
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13706?src=pr) is **84.54%**

> Merging [#13706](https://codecov.io/gh/pydata/pandas/pull/13706?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13706   diff @@
==========================================
  Files           141        141          
  Lines         51185      51164    -21   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43275      43255    -20   
+ Misses         7910       7909     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [506520b...d097bab](https://codecov.io/gh/pydata/pandas/compare/506520bd35331aa82db50686c07d96594cac0c10...d097babf8253d3e91c468f7be6cb12a724481d16?src=pr)
 No real objections from me, although the functions (looking at the length of the code) seems to do a lot more than the cookbook example? 
SO gives no results for a search for pivot_annual, a good sign :-)

Deprecate `isleapyear` as well?
 I agree should deprecate isleapyear but add on as an accessor for Timestamp/DTI and .dt
which should be straightforward to do
 @jreback There were still some unresolved comments I think (deprecating isleapyear as well)
 @jorisvandenbossche ahh yes, I mean to make an issue about that.
 I wii subbmit new PR (to deprecate `isleapyear` and adding the same proprty).

one minor pount is the property name. `Timestamp` has property like `.is_year_end`, numpy datetime.c has `is_leapyear` on the other hand. Which is prefereble, `is_leap_year` or `is_leapyear`?
 no problem!
 I think `is_leapyear` is fine

`leapyear` is really the common english word (I see though already getting inconsistent as we should really do `is_year_leap`)?
