Generate documentation for error IDs

[nicolasstucki]

We should generate the documentation @ckipp01 collected in index in an official part of the Scala 3 documentation. This list should be up to date with all the errors listed in ErrorMessageID.scala. A good way to make sure that the documentation is complete would be to make the IDs hold the explanations of the error messages. Not sure if those would be the same explanations that we have in Message.scala, if it is the case it would be simpler to maintain.

Originally posted by @nicolasstucki in #14965 (comment)

[pikinier20]

We could reuse the tool that generates the index in Chris's repo and generate a static site that will be present in language reference

[nicolasstucki]

Not sure if that is the most maintainable approach. We should also have something that forces us to create those examples when adding new error messages.

[pikinier20]

Another idea would be to add docstring to each case in https://github.com/lampepfl/dotty/blob/73e635143bf68c388e4dc4d88054eca1dc3a09ea/compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala which would have snippet with example. Generating documentation from this file with snippet checking would build a page where you could see an example with error message for each error ID.