docs: add z_bytes example

[wyfo]

Based on #322

[github-actions]

PR missing one of the required labels: {'new feature', 'dependencies', 'enhancement', 'breaking-change', 'bug', 'internal', 'documentation'}

[Mallets]

An example of Protobuf serialization is missing and it should be added as per eclipse-zenoh/zenoh#1418

[wyfo]

Protobuf requires to install additional dependencies, and contrary to Rust, this is not done automatically. Also, protobuf for Python is not like Prost, it requires to generate code and compile binary from a proto file.
All this process is quite heavy, and not automatic, that's why I've not included protobuf

[Mallets]

Protobuf is a widely-used serialization library and we have received many questions on how to use Protobuf with Zenoh.
We should provide an example.

[Mallets]

I'm going to approve and merge this PR so as to already provide a set of examples for serialization/deserialization with ZBytes. However, a follow-up PR should following including Protobuf.