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?

[MaryaBelanger]

It seems like Runtime errors can move to the types system page too, under Runtime checks. What do you think? It can almost replace the existing section 1:1 (though I'd try to shorten it a bit, remove the "Fix: ..." section header, etc).

Edit: It's a little more involved than that; seems like the Type inference section that follows Runtime checks on the Type system page repeats a lot of information from the Sound problems section. I think it should still move to the Type system page, but we'll have to be more intentional than just copy-pasting the whole thing (especially in the Type argument inference subsection, seems like a lot of that in the same as the "Fix" section under Runtime errors)