Updates all links to the Flutter wiki to point to their new location in the flutter/flutter repository. (The sole exception is a link to a doc that doesn't have a final home yet, and is linked from legacy code anyway so doesn't really need to be updated.)

While touching the PR template, makes a few minor improvements:
- Removes the breaking change discussion that doesn't apply to this repository, as breaking changes are handled totally differently for packages (and is covered by the link to docs about Dart versioning).
- Adds text and a link to reflect the fact that some PRs can be changelog-exempt.