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

[parlough]

Page: https://dart.dev/guides/language/sound-problems#the-covariant-keyword
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.

[parlough]

\cc @MaryaBelanger Any thoughts on where we can/should document the covariant keyword?

Perhaps https://dart.dev/language/type-system#methods and/or on https://dart.dev/language/methods?

The existing location has been outside of the normal language docs, resulting in people often not knowing the keyword exists. This is perhaps partially intentional as it moves a static error to runtime. So wherever we move it, perhaps a warning is needed alongside?