Document covariant elsewhere then retire "Fixing common type problems" page

[parlough]

Page: https://dart.dev/guides/language/sound-problems#troubleshooting
Source: https://github.com/dart-lang/site-www/blob/main/src/content/guides/language/sound-problems.md

The page was introduced in the context of sound Dart being new and before the diagnostic messages were well documented. Now the warnings documented here (and many more) have consistent documentation on the Diagnostic messages page.

The only parts not covered by that page is the section on runtime errors and the covariant keyword. These docs need a new home before retiring the page, or at least the covariant docs do. That probably belongs in the language docs anyway.