Integer NA docs
Closes https://github.com/pandas-dev/pandas/issues/22003

In the issue @chris-b1 said

> More clear separation from numpy dtype - worry that 'int64' vs 'Int64' will be especially confusing for new people? Consider a different name altogether? (NullableInt64?)

Do people have thoughts on that?

Earlier @jreback suggested putting all this in `missing_data.rst`, rather than a new page. But that page is already quite long. I think it's OK to spread out a little.

I tried to track down references to missing data and integers in the docs to add links to integer-NA. Let me know if you see any I missed.

Note: this uses `pd.array` and some types added to the API in https://github.com/pandas-dev/pandas/pull/23581, so not all the examples will run.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23617?src=pr&el=h1) Report
> Merging [#23617](https://codecov.io/gh/pandas-dev/pandas/pull/23617?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e4cd00bc0fbf3e88c3efb21151c33cdb9d1aab0a?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/23617/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23617?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23617   +/-   ##
=======================================
  Coverage   31.88%   31.88%           
=======================================
  Files         166      166           
  Lines       52425    52425           
=======================================
  Hits        16717    16717           
  Misses      35708    35708
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `30.29% <ø> (ø)` | :arrow_up: |
| #single | `31.88% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23617?src=pr&el=continue).
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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23617?src=pr&el=footer). Last update [e4cd00b...0c9995f](https://codecov.io/gh/pandas-dev/pandas/pull/23617?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Maybe `IntNA64`? Perhaps not terribly readable but aligns with how we've referenced it @jreback any objections to having this in it's own document? I think our docs would benefit from breaking up some of the extremely long pages :) no this is ok
though we should merge the pd.array change first no? Sure, then we can check the doc build. Fixed the merge conflicts. @TomAugspurger can you merge master Will do. Waiting for pandas.array to be merged first, since I use it here.

On Fri, Dec 28, 2018 at 2:35 PM Jeff Reback <notifications@github.com>
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> @TomAugspurger <https://github.com/TomAugspurger> can you merge master
>
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 @TomAugspurger right hahah I approved so went to dependent PR. ok then, this looks ok. https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=6029 some pesky warnings this is passing now. @TomAugspurger did you have any other changes here? Thanks for fixing it up. 