DOC: Add pandas_path to the accesor list in the documentation
Added the [`pandas_path`](https://github.com/drivendataorg/pandas-path/) package that I just released to the list of third-party accessor extensions.

As a bonus, added a Description column for the existing libraries.
@datapythonista Cool, I added a longer description to the "Data cleaning" section, which seemed the most relevant. That make sense? Thanks @pjbull 