DOC/REF: Rolling/Expanding/EWM docstrings
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them


- Create a new `window/doc.py` file for common template
- Using `@doc` decorator instead of `@Appender`
- Standardize descriptions and sections

<img width="1255" alt="Screen Shot 2021-01-16 at 5 06 52 PM" src="https://user-images.githubusercontent.com/10647082/104828385-c7601680-5860-11eb-99b5-e73357719348.png">
 

cc @jorisvandenbossche @simonjayhawkins if you have any comments ~I might need a precursor PR to align function signatures.~

~@jreback thoughts on all functions having a signature of `def func(*args, <keyword_only_arguments>, **kwargs)`?~

~Though `*args`, `**kwargs` may never be used.~

EDIT: Nevermind hmm somthing going wrong on the docs build Fixed the doc build failing tests and all green.

Refactored now to omit empty sections. ahh can you rebase cc @simonjayhawkins @jorisvandenbossche if any other comments. Going to merge and can address any follow ups in another PR