Feature: Terrain Preview and simple Elevation Query

[okimiko]

This PR will add

• a preview for terrain data (in addition to the current raw data preview)
  • highlight color is based on the encoding
• a simple elevation api based on served terrain data
  • includes a link on the index page
  • for details, see endpoints.rst

Both require configured encoding (see config.rst).

In general this adds just some eye candy and an "inspect" function for terrain like the geojson endpoint.