Add example using shared variables

[aloctavodia]

Currently, the only "example" is a test :-(

https://github.com/pymc-devs/pymc-bart/blob/main/tests/test_bart.py#L43-L58

[juanitorduz]

I think I could help with this one as I have been working with BART for out-of-sample predictions lately. I was thinking about using the bikes example. Should it be a different notebook or an extension of the existing one? https://github.com/pymc-devs/pymc-examples/blob/main/examples/case_studies/BART_introduction.ipynb

[aloctavodia]

Thanks! Both options are good.

[juanitorduz]

Here is my first proposal: pymc-devs/pymc-examples#507