Update experimentation best practices

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Update/document best practices for experimentation testing with some of the newer suggestions we have discussed.

1. When testing on staging rollout(10%) should be set to a low threshold to expose double segmentation errors.
2. Feature specs should be present for cross page experiments
3. Specs should use the :experiment_tracking metadata so that we ensure/document tracking data is sent
4. Use :only_assigned for experiment blocks that are not experiment entry points.

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