SparseArray not in arrays module - inconsistent with IntegerArray, StringArray, etc.
#### Code Sample, a copy-pastable example if possible

```python
In [1]: import pandas as pd

In [2]: pd.__version__
Out[2]: '0.26.0.dev0+1563.g1feefc692'

In [3]: pd.SparseArray
Out[3]: pandas.core.arrays.sparse.array.SparseArray

In [4]: pd.arrays.SparseArray
Out[4]: pandas.core.arrays.sparse.array.SparseArray

In [5]: pd.arrays.IntegerArray
Out[5]: pandas.core.arrays.integer.IntegerArray

In [6]: pd.IntegerArray
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-6-12476104dd13> in <module>
----> 1 pd.IntegerArray

C:\Code\pandas_dev\pandas\pandas\__init__.py in __getattr__(name)
    246             return type(name, (), {})
    247
--> 248         raise AttributeError(f"module 'pandas' has no attribute '{name}'")
    249
    250

AttributeError: module 'pandas' has no attribute 'IntegerArray'

In [7]: pd.arrays.StringArray
Out[7]: pandas.core.arrays.string_.StringArray

In [8]: pd.StringArray
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-8-86553ff3c48c> in <module>
----> 1 pd.StringArray

C:\Code\pandas_dev\pandas\pandas\__init__.py in __getattr__(name)
    246             return type(name, (), {})
    247
--> 248         raise AttributeError(f"module 'pandas' has no attribute '{name}'")
    249
    250

AttributeError: module 'pandas' has no attribute 'StringArray'
```
#### Problem description

I discovered this while working on #30628 .  The docs for `SparseArray` are at the top level (https://dev.pandas.io/docs/reference/api/pandas.SparseArray.html), while the docs for `IntegerArray` (https://dev.pandas.io/docs/reference/api/pandas.arrays.IntegerArray.html), `StringArray`(https://dev.pandas.io/docs/reference/api/pandas.arrays.StringArray.html), etc. are at the `pandas.arrays` level.

In the code `SparseArray` is at both levels, but `IntegerArray`, `StringArray`, etc. is only at the `arrays` level.

#### Expected Output

Unsure.  

It seems that this should be consistent.  Options are:
1. Put all `*Array` classes at top level, and document them that way. (i.e., use the pattern currently used for `SparseArray`).  That would involve code and documentation changes for all of the arrays except `SparseArray`.
2. Put all `*Array` classes at both levels (like `SparseArray`), but document them at the `pandas.arrays` level (like `IntegerArray` and `StringArray`).  That would involve code changes for all of the arrays, and doc changes for `SparseArray`.
3. Put all `*Array` classes only at the `pandas.arrays` level and document them all that way.  That would involve only changing code and docs for `SparseArray` and leaving the others alone.

It's not clear to me which is preferred.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : 1feefc69241a8944bbdf3e7cd042336ef9554be7
python           : 3.7.3.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 158 Stepping 13, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 0.26.0.dev0+1563.g1feefc692
numpy            : 1.17.4
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 42.0.2.post20191203
Cython           : 0.29.14
pytest           : 5.3.2
hypothesis       : 4.54.2
sphinx           : 2.3.0
blosc            : None
feather          : None
xlsxwriter       : 1.2.6
lxml.etree       : 4.4.2
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : 7.10.2
pandas_datareader: None
bs4              : 4.8.1
bottleneck       : 1.3.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.2
matplotlib       : 3.1.1
numexpr          : 2.7.0
odfpy            : None
openpyxl         : 3.0.2
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.3.2
s3fs             : None
scipy            : 1.3.2
sqlalchemy       : 1.3.11
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.6
numba            : 0.46.0

</details>
