add link to terrain tileset. (#92)

* add link to terrain tileset.

* fix link

Author: bdon

basemaps/downloads.md

@@ -27,3 +27,9 @@ A mirror in the AWS `us-west-2` is available on [Source Cooperative (beta)](http
 To download a cutout of a specific region, rather than the entire world map, see the CLI's [extract command](/pmtiles/cli#extract).
 
 If you don't need all 16 zoom levels of detail, use the `--maxzoom` option of `pmtiles extract`. Each additional zoom level roughly doubles the size of the file.
+
+## Terrain
+
+The [AWS Registry of Open Data](https://registry.opendata.aws/terrain-tiles/) hosts Terrarium-encoded RGB tiles generated by the [Joerd](https://github.com/tilezen/joerd) project. Below is a link of zooms 0 to 12 of this tileset, provided as-is. See the [Joerd docs](https://github.com/tilezen/joerd/blob/master/docs/attribution.md) for attribution requirements.
+
+[terrarium-z12.pmtiles](https://r2-public.protomaps.com/protomaps-sample-datasets/terrarium-z12.pmtiles) [(preview)](https://pmtiles.io/?url=https%3A%2F%2Fr2-public.protomaps.com%2Fprotomaps-sample-datasets%2Fterrarium-z12.pmtiles#map=0.7/0/0)

pmtiles/maplibre.md

@@ -77,7 +77,7 @@ For raster tiles you'll just need to change the `type` of your source to `raster
 }
 ```
 
-Protomaps also distributes terrain tilesets in the [Terrarium RGB encoding](https://github.com/tilezen/joerd/blob/master/docs/formats.md). These have a special source type in MapLibre GL:
+Protomaps also [distributes terrain tilesets](/basemaps/downloads#terrain) in the [Terrarium RGB encoding](https://github.com/tilezen/joerd/blob/master/docs/formats.md). These have a special source type in MapLibre GL:
 
 
 ```json{5}