Link to and/or excerpt offline tile creation guide with QGIS

Another excellent resource from @florianm: https://forum.getodk.org/t/generate-offline-background-imagery-for-odk-collect-mbtiles/31200 🙏

Start with the linked forum guide about generating offline background imagery for ODK Collect and review the documentation structure to find the appropriate location. Add a link to the guide or an excerpt explaining the workflow, then verify that the resulting documentation makes the resource easy to find.

Written by the indexing model from the issue text.