Move statespace example notebooks over to `pymc-devs/pymc-examples` repo

[jessegrabowski]

There are currently 5 statespace example notebooks in this repo:

• Exponential Trend Smoothing
• Making a Custom Statespace Model
• SARMA Example
• VARMAX Example
• Structural Timeseries Modeling

These would have a lot more visibility and be a lot more useful to people over on the pymc-examples repo.

A few of them are a bit underdeveloped (the VARMAX one in particular) but I think for the most part they could be moved over as-is.