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.
The metaprogramming to create all the Int* dtypes is causing issues with sphinx / numpydoc

![screen shot 2019-01-04 at 3 37 06 pm](https://user-images.githubusercontent.com/1312546/50712327-b441cb80-1036-11e9-9224-26a0293ca581.png)
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24626?src=pr&el=h1) Report
> Merging [#24626](https://codecov.io/gh/pandas-dev/pandas/pull/24626?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cb31b2b0912c99e40b22a26e598e640548ea8f14?src=pr&el=desc) will **decrease** coverage by `49.37%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24626/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24626?src=pr&el=tree)

```diff
@@             Coverage Diff             @@
##           master   #24626       +/-   ##
===========================================
- Coverage   92.38%      43%   -49.38%     
===========================================
  Files         166      166               
  Lines       52395    52395               
===========================================
- Hits        48403    22534    -25869     
- Misses       3992    29861    +25869
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `?` | |
| #single | `43% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24626?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/latex.py](https://codecov.io/gh/pandas-dev/pandas/pull/24626/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvbGF0ZXgucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/core/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/24626/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY2F0ZWdvcmljYWwucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/io/sas/sas\_constants.py](https://codecov.io/gh/pandas-dev/pandas/pull/24626/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfY29uc3RhbnRzLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/plotting.py](https://codecov.io/gh/pandas-dev/pandas/pull/24626/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvcGxvdHRpbmcucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/24626/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvY29udmVydGVyLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/io/formats/html.py](https://codecov.io/gh/pandas-dev/pandas/pull/24626/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvaHRtbC5weQ==) | `0% <0%> (-99.37%)` | :arrow_down: |
| [pandas/core/groupby/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/24626/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9jYXRlZ29yaWNhbC5weQ==) | `0% <0%> (-95.46%)` | :arrow_down: |
| [pandas/io/sas/sas7bdat.py](https://codecov.io/gh/pandas-dev/pandas/pull/24626/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXM3YmRhdC5weQ==) | `0% <0%> (-91.17%)` | :arrow_down: |
| [pandas/io/sas/sas\_xport.py](https://codecov.io/gh/pandas-dev/pandas/pull/24626/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfeHBvcnQucHk=) | `0% <0%> (-90.15%)` | :arrow_down: |
| [pandas/core/tools/numeric.py](https://codecov.io/gh/pandas-dev/pandas/pull/24626/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvbnVtZXJpYy5weQ==) | `10.44% <0%> (-89.56%)` | :arrow_down: |
| ... and [124 more](https://codecov.io/gh/pandas-dev/pandas/pull/24626/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24626?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24626?src=pr&el=footer). Last update [cb31b2b...a5ac93d](https://codecov.io/gh/pandas-dev/pandas/pull/24626?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24626?src=pr&el=h1) Report
> Merging [#24626](https://codecov.io/gh/pandas-dev/pandas/pull/24626?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cb31b2b0912c99e40b22a26e598e640548ea8f14?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24626/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24626?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24626   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files         166      166           
  Lines       52395    52395           
=======================================
  Hits        48403    48403           
  Misses       3992     3992
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24626?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/array\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/24626/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2FycmF5Xy5weQ==) | `100% <ø> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/24626/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.62% <ø> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/24626/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.92% <ø> (ø)` | :arrow_up: |
| [pandas/core/arrays/sparse.py](https://codecov.io/gh/pandas-dev/pandas/pull/24626/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3NwYXJzZS5weQ==) | `92.17% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24626?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24626?src=pr&el=footer). Last update [cb31b2b...d650fa1](https://codecov.io/gh/pandas-dev/pandas/pull/24626?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 You're correct about the heading levels. I'll fix those. thanks!