Doc: clarify why constrained flows are needed.

[gil2rok]

Clarify why constrained flows are needed.

In my opinion, this example does not sufficiently motivate why constrained flows are needed. This makes it confusing for a new user, especially since working examples are how they often learn.

To remedy this, the example now explicitly mentions the support of the beta distribution and how we need to ensure our flow respects it.

Also rename file and change title to use the more common "constrained" transform instead of "bounded" transform. See https://mc-stan.org/docs/reference-manual/transforms.html.