For reference rust has a error codes index, clicking on an error code gives examples and explanations.

Should we do something similar? How could we implement it.

Perhaps extracting rendered scaladoc is good enough, I could also imagine using infrastructure similar to Scala CLI where documentation for using directives is generated using annotations directly on the source code of the error message.

Ive also seen a suggestion that the compiler itself could explain errors from the command line, rather than hosting a website see here

Also this brings back the issue of should we remove the capability to report an error/warning from just a string (i.e. enforce ErrorMessageID usage)

other discussion: https://contributors.scala-lang.org/t/revisiting-dotty-diagnostics-for-tooling/5649