ENH: Implement weighted rolling var and std
- [x] closes #26597
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry


Used West's online [algorithm](https://dl.acm.org/citation.cfm?id=359153). [Here](http://www.nowozin.net/sebastian/blog/streaming-mean-and-variance-computation.html) is an explanation of the algorithm and link to pdf of the paper. Tested implementation with `win_type="boxcar"` comparing it with the result of `Rolling.std`.

Additionaly, `_get_window` function is split into two. `_get_kwargs` function is used to split kwargs for window function and rolling function (since std and var takes an additional `ddof` argument).

Shared docs for `std` and `var` are moved to be used with `Window` functions

@jreback could you review this? can you merge master and will review @jreback merged can you merge master and will have another look @jreback merged @ihsansecer I think this is good; can you merge master and fix up conflict? thanks @ihsansecer nice patch.

can you do a followup to make these nogil (@jbrockmendel recently added most window functions like this, e.g. see roll_sum) @jreback sure but I have been a bit busy these days. I will do it when I have time.