DOC: Add SparseDataFrame and SparseSeries to the API docs
We have some references them that don't resolve currently, since the classes aren't there.

Unfortunately it looks like adding `SparseDataFrame` causes a failure like

```
ValueError: %(klass)s.notna : boolean inverse of isna is not a item name
[70] > /Users/taugspurger/Envs/pandas-dev/lib/python3.6/site-packages/pandas/doc/sphinxext/numpydoc/docscrape.py(210)parse_item_name()
-> raise ValueError("%s is not a item name" % text)
(Pdb++) Building HTML failed.
```