Implement and document linking a lib with the client

[ataffanel]

Before k-config it used to be possible to easily link add a static lib to be linked in the Crazfile firmware. This allowed, among other thing, to expand the firmware with Rust code.

It does not seems easy to properly add static libs to kconfig. This needs to be investigated and documented.

Related to https://github.com/orgs/bitcraze/discussions/1236