Schematics are available https://the-analog-thing.org/docs/dirhtml/rst/schematics/ though usually Open Source Hardware refers to releasing the original design files, including schematic and board layout, rather than just PDFs of the schematic: https://www.oshwa.org/definition/

If you are looking for documentation, try clicking the prominent "read the docs" link. Once you do so, you'll notice "THAT schematics" is the very first link after the overview.