DOC: Add SparseDataFrame and SparseSeries to the API docs

[TomAugspurger]

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.

[jorisvandenbossche]

I would first like to see a way to somehow limit the number of api pages (eg a way to only build pages for methods that are overwritten) before adding such a big new chunk to the api pages.

[TomAugspurger]

Yeah, we should only include the methods we actually refer to.