Upgrade stan code in docs to 3.0.2

[javipus]

Describe the bug The stan code in this example notebook included in the docs does not work with v3.0.2 for several reasons:

1. The library should now be imported as import stan.
2. The class StanModel has been deprecated.

To Reproduce Run the notebook.

Expected behavior The code should run without errors.

Additional context NA

Proposed solution Follow the example in stan's documentation.

[ahartikainen]

We might want to keep also example for pystan 2.x, but also add a new example for 3.x.