Array api docs
Closes 
https://github.com/pandas-dev/pandas/issues/24066

cc @datapythonista @jorisvandenbossche there are a few closely related changes here.

* removed api.scalars. I think it's more useful to present "Here are all pandas extensions", and then sub-divide into array / scalar / dtype. I think arrays are the most prominent of these, so I I've named the file `api/arrays.rst`
* Moved `pd.array` to api/arrays.rst
* Moved Categorical from api/series.rst to api/arrays.rst
* Re-written the accessor section and moved / left those in api/series.rst
* Added DatetimeArray (the original point of this PR)

---

I have quite a few warnings to cleanup, but wanted a +1 on the general reorganization before putting more time into it.