Issue #352: add z-axis to item spec for bbox and geometry

[joshfix]

Related Issue(s): #35

Proposed Changes:

1. Add documentation to the item spec in support of z-axis for geometry and bbox values.

PR Checklist:

• This PR has no breaking changes.
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.
• API only: I have run npm run generate-all to update the generated OpenAPI files.

[m-mohr]

@joshfix I think there are potentially more places, which need an update:

• api-spec.md, line 62 is about bbox and is missing the z-axis ("[west, south, east, north]")
• To align between collection and item, I'd suggest to also update the collection-spec.md around line 67 (height vs. elevation).

Should a CHANGELOG entry be added? Not because it's really a change, but to make implementers aware of that they may have missed implementing z-axis support?

[cholmes]

Looks great, thanks!