Update docs with Firefox + GH pages issue and Mapterhorn links. (#122)

Author: bdon

basemaps/downloads.md

@@ -40,6 +40,12 @@ If you don't need all 16 zoom levels of detail, use the `--maxzoom` option of `p
 
 ## Terrain
 
+### Mapterhorn
+
+The [Mapterhorn](http://mapterhorn.com) project distributes Terrarium-encoded RGB tiles aggregated from multiple global data sources in PMTiles format. Download the full `.pmtiles` or get a partial download with `pmtiles extract` at the [Mapterhorn Data Access documentation](https://mapterhorn.com/data-access/).
+
+### Tilezen Joerd (legacy)
+
 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/cloud-storage.md

@@ -277,6 +277,10 @@ pmtiles upload file.pmtiles path/in/bucket/key.pmtiles --bucket=s3://ORGANIZATIO
 
 GitHub pages supports repositories up to [1 GB](https://docs.github.com/en/pages/getting-started-with-github-pages/about-github-pages#usage-limits). If your PMTiles file fits, it's an easy way to host.
 
+:::warning
+PMTiles archives hosted on GitHub Pages currently do not load in Firefox. See [this GitHub issue](https://github.com/protomaps/PMTiles/issues/584) for the bug description and workaround.
+:::
+
 ### Scaleway
 
 [Scaleway Object Storage](https://www.scaleway.com/en/object-storage/) only supports HTTP/1.1.