You said “the links on the right for documentation no longer work” — can you copy and paste the link here, or post a screenshot? The links seem to work fine for me. A few months ago we migrated the old support site (opensprinkler.freshdesk.com) to openthings.freshdesk.com and some old links may have been updated. But the ones on the page you mentioned all seem up-to-date.
Also, the page you posted is for OpenSprinkler Pi, is that what you are looking for? If you are looking for the firmware compilation instructions for OS 3.0, it’s here:
https://openthings.freshdesk.com/support/solutions/articles/5000165132-how-to-compile-opensprinkler-firmware