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?
