Predictive back transition breaking change announcement

[justinmc]

This PR spawned out of flutter/flutter#165832 (comment). That PR will change the default page transition and turn on predictive back by default. While there is no API breakage, users may encounter test breakages in tests that indirectly depend on the default page transition and/or its duration. Several of these kinds of breakages were encountered in flutter/flutter#165832 in the framework tests, customer tests, and Google tests.

This PR should explain:

• What predictive back is.
• How to make sure an app supports predictive back.
• What the new Fade Fowards page transition looks like.
• How to manually change the page transition back to the old one.
• How to write tests that don't depend on the page transition and its duration.

Issues fixed by this PR (if any): n/a

PRs or commits this PR depends on (if any): flutter/flutter#165832

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