DOC: Missing PandasArray methods in API
We've got warning in sphinx because we don't have the `PandasArray` methods in the API:
```
/home/mgarcia/src/pandas/doc/source/api/generated/pandas.arrays.PandasArray.all.rst: WARNING: document isn't included in any toctree
/home/mgarcia/src/pandas/doc/source/api/generated/pandas.arrays.PandasArray.any.rst: WARNING: document isn't included in any toctree
/home/mgarcia/src/pandas/doc/source/api/generated/pandas.arrays.PandasArray.kurt.rst: WARNING: document isn't included in any toctree
/home/mgarcia/src/pandas/doc/source/api/generated/pandas.arrays.PandasArray.max.rst: WARNING: document isn't included in any toctree
/home/mgarcia/src/pandas/doc/source/api/generated/pandas.arrays.PandasArray.mean.rst: WARNING: document isn't included in any toctree
/home/mgarcia/src/pandas/doc/source/api/generated/pandas.arrays.PandasArray.median.rst: WARNING: document isn't included in any toctree
/home/mgarcia/src/pandas/doc/source/api/generated/pandas.arrays.PandasArray.min.rst: WARNING: document isn't included in any toctree
/home/mgarcia/src/pandas/doc/source/api/generated/pandas.arrays.PandasArray.prod.rst: WARNING: document isn't included in any toctree
/home/mgarcia/src/pandas/doc/source/api/generated/pandas.arrays.PandasArray.sem.rst: WARNING: document isn't included in any toctree
/home/mgarcia/src/pandas/doc/source/api/generated/pandas.arrays.PandasArray.skew.rst: WARNING: document isn't included in any toctree
/home/mgarcia/src/pandas/doc/source/api/generated/pandas.arrays.PandasArray.std.rst: WARNING: document isn't included in any toctree
/home/mgarcia/src/pandas/doc/source/api/generated/pandas.arrays.PandasArray.sum.rst: WARNING: document isn't included in any toctree
/home/mgarcia/src/pandas/doc/source/api/generated/pandas.arrays.PandasArray.var.rst: WARNING: document isn't included in any toctree
```

I guess those are public and should be added. But I'm not quite sure where we should have them, we have some related stuff in two different places:

- https://pandas-docs.github.io/pandas-docs-travis/api/series.html#arrays
- https://pandas-docs.github.io/pandas-docs-travis/api/extensions.html

Not sure if we need to reorganize those. @TomAugspurger 