provide more details about z_timestamp_t

[DenisBiryukov91]

Describe the release item

Currently we provide no information about what z_timestamp_t is, making it completely useless for c-users. I think it would probably make sense to provide at least some context (like that z_timestamp_t::time is a time since Unix Epoch in ntp64 format) and maybe a couple of api to convert from/to now available z_time_t (or simply getting/converting number of seconds/ms/us since Unix Epoch).

[Mallets]

Timestamp definition in Zenoh: https://docs.rs/uhlc/latest/uhlc/