DOC: Fix creation of [source] links in the doc creation
See this issue: https://github.com/pandas-dev/pandas/issues/23046

A simple fix is to add `inspect.unwrap(obj)` before `getsourcefile`. This follows the `__wrapped__` attributes of the functions "all the way to the bottom" and then returns the object found.

One problem with this is that `unwrap` function was only introduced in Python 3.4. This means that compiling the documentation will now fail on Python versions older than that. I do not know if this is acceptable.