Improve scaphandre to be usable as a library/crate

[bpetit]

Problem

Scaphandre has mostly been thought as a cli tool/daemon until now.
It is usable as a crate but the track is not tagged.

Solution

Improve code documentation and add a tutorial in the docs to use scaph from a rust program.

Alternatives

Not an alternative but a potential helper: write an exporter that should be the privileged interface to get metrics from scaph.

Additional context

Thoughts on this are very welcome. We may have a look at popular crates too to identify the best practices to make crate's integration smoother.