The old site supported api.dart.dev/stable/<page in the latest version>
links. These currently don't work, but the correct link omits the
`/stable/` path component.

Even if we add redirects to make the old URLs work, we still want to
update sources with the canonical URLs.

CoreLibraryReviewExempt: Documentation change only.
Fixes: https://github.com/dart-lang/sdk/issues/57110
Change-Id: I0b20bfa875db5ce62362a02e884d7d43b0efc73e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/395961


Commit-Queue: Ivan Inozemtsev <iinozemtsev@google.com>
Reviewed-by: Ivan Inozemtsev <iinozemtsev@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>
Auto-Submit: Alexander Thomas <athom@google.com>