Make it easy to navigate across documentation for SDK versions

[jamesderlin]

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In addition to https://github.com/dart-lang/api.dart.dev/issues/53, something else to consider is to make it easy to navigate across versions from within the documentation pages themselves. That would improve the situation for all existing links in the wild.

IMO, Python's documentation handles this very well. For example: https://docs.python.org/3/library/subprocess.html

I don't know if that's possible. I think generated dartdocs are self-contained, so maybe not by dartdoc, but maybe the hosting site can do something special?