Stabilize and document connectivity events

[p-avital]

Describe the release item

With the next release, we'd like to add support for connectivity events in Zenoh Pico.

However, to allow code portability, Zenoh Pico and Zenoh C should support them through the same API. As the current API is an unstable and undocumented set of keyexprs that can be listened to, we need to stabilize and document it before we can add support for it to Pico.