Update for schema v15

[dbrgn]

Checklist:

• [x] Update schema docs (https://github.com/SpaceApi/website/pull/139)
• [x] Update example (https://github.com/SpaceApi/website/pull/139)
• [ ] Update mention of nullability
• [x] Test that validator works with v15
• [x] Release schema v15 (https://github.com/SpaceApi/schema/pull/117, https://github.com/SpaceApi/schema/pull/118)

[rnestler]

What is meant with "Update mention of nullability"?

[dbrgn]

Probably the sentence here https://spaceapi.io/docs/:

Most types are not nullable. That means that they may not contain the value "null", but they may be left away if they're not required.

In v14, the state.open field was nullable, since v15 no field is nullable anymore. Suggestion for new wording:

Since v15, no more schema fields are nullable. That means that they may not contain the value "null", but they may be left away if they're not required.

[rnestler]

I think in the future we'd want to handle null the same as absent.