Split serialisation topic and better organise

[hamishwillee]

https://mavlink.io/en/guide/serialization.html

This doc is primarily about the serialisation format - ie it is most useful for people implementing their own parsers.

The top however has a user example about the serialisation API.

I think the user info should go into the "Using" examples, and we should link to it from this doc. This doc should probably also move into a section for mavlink-library implementer developers.

[hamishwillee]

Will be fixed by #80