[Sound null safety] Language feature docs

Evaluation of potential changes due to sound null safety: https://github.com/dart-lang/sdk/issues/49530

(READY) Mentions of optional/unsound null safety across the site need to be removed. There may be some cases where indiscriminately removing related text would make the content confusing, so evaluate the context for each instance (why did it need to be mentioned before, is a caveat still applicable, etc).
(NEEDS FURTHER EVAL) The pages on null safety (listed under Evaluation) need to be dealt with. Especially /migration-guide, /unsound-null-safety, and /faq will likely be completely removed, but each needs to be evaluated for content that may still be useful to users (don't indiscriminately remove them).

The likely solution is to create a new page under "Types" about nullable types to include some information, and separate out useful, non-version-qualified content (i.e. "before null safety/Dart 2...." kind of qualifiers) from /null-safety and /understanding-null-safety and either put that info onto new pages, or consolidate with other content. Basically we need to pick apart these pages line by line to keep what's relevant and discard what isn't.

PRs that already addressed sound null safety pre-2.19:

Pages on null safety:

Other mentions of optional null safety:

Things that need to be added somewhere:

Libraries using per-library language version markers selecting versions less than 2.12 (e.g. // @dart=2.9 will be disallowed).

Code using an SDK constraint where the min-constraint isn't at least 2.12 (e.g. sdk: ">=2.7.0" will be disallowed and won't resolve).

dart-lang / site-www