Technical documentation

[vincentfretin]

Some interesting technical documentation are buried in closed PR, like this comment about the Box<Arc<Session>>
and how it's created in janus-plugin-rs and dropped.

The incref/decref of jansson structs and libcstring created on the Rust side or C++ side are also really interesting, see the JanssonValue struct and LibcString struct

Put that info in a technical doc in this repo.