update the url for specification documents for the MBTiles tileset format

This url: https://docs.mapbox.com/help/how-mapbox-works/#mbtiles

just points to the general MapBox help for me. I'm not sure if you want to point to the mbtiles page in the glossary (https://docs.mapbox.com/help/glossary/mbtiles/) or somewhere else.

Search the repository's specification documents for the outdated Mapbox help URL and inspect the linked MBTiles glossary page as the proposed replacement. Update the reference to the agreed MBTiles documentation destination, then verify that no specification link still points to the general help page.

Written by the indexing model from the issue text.