the page being linked to is currently part of the stan-dev.github.io website - cf this Website page on Guide to Stan's Warnings
people come to Discourse looking for answers, but this is something that should be in the Stan docs - there’s sections in both the Stan User’s Guide (advice) and Reference Manual (definitions), also in the CmdStan guide 17 diagnose: Diagnosing Biased Hamiltonian Monte Carlo Inferences | CmdStan User’s Guide