Review if StarPos should be a top-level mandatory property

[Athanasius]

I'm mostly here thinking about documenting it, in the general schemas README, as a mandatory augmentation for any event/schema where we can reasonably expect the data to be available.

I'm assuming listeners always want this in case they only get, e.g. an ApproachSettlement EDDN message (sender might have had issues with things like FSDJump, or maybe this listener restarted and only caught the in-system events).

Whilst it might be tempting to say "Always add StarPos if you have SystemAddress" I can also see listeners wanting SystemAddress adding if not already present. In general the senders should be able to always remember this from one of Location, FSDJump, CarrierJump, Docked events.

NB: Possible exception needs to be made for where data can be CAPI-sourced, so these augmentations so far as the top-level documentation is concerned may well need to be optional. event/schema-specific documentation may make them individually mandatory if that makes sense.