DOC: Add STUMPY to pandas ecosystem
#### Location of the documentation

[Pandas Ecosystem - Stats & ML](https://pandas.pydata.org/docs/ecosystem.html#statistics-and-machine-learning)

#### Documentation problem

This isn't a problem, just an addition to the current ecosystem listing.

#### Suggested fix for documentation

This is a simple addition to the `ecosystem.rst` file for adding [STUMPY](https://github.com/TDAmeritrade/stumpy) to the existing docs