Publish Elevation Extension

[jyutzler]

Publish the Elevation Extension (https://github.com/opengeospatial/geopackage-elevation) as an official GeoPackage extension. Depending on timing, it could be added directly to the document (e.g., Annex F.11) or published separately.

[jyutzler]

At the TC, we voted to add this to the specification. I will add annexes F.11 and F.12 and update the references in the other github repository to point here. I am leaving this ticket open until I can finish the change.

[jyutzler]

Fixed by #248, #250, and some additional manual edits.