Improve packet signing documentation

[hamishwillee]

Packet signing is currently primarily about the C implementation. It needs to keep that information, but be more generic/reusable. That way we can have doc for people who want to understand how things work and implement their own generator, and separate docs on the detail for each generated MAVLink library.

Background for signing is here: https://docs.google.com/document/d/1ETle6qQRcaNWAmpG2wz0oOpFKSF_bcTmYMQvtTGI8ns/edit#heading=h.r1r08t7lr2pc

[hamishwillee]

Note: If possible, link to Clojure docs on signing: WickedShell/clj-mavlink#2