Move `platforms.yaml` to root

Issue:

Each model yaml for compilation includes a platforms.yaml. Thus, there could be a different versions of this yaml file in each folder. The platforms.yaml should be more central and the same one should be used by all the model's.

Solution:

The platforms yaml can be put in the root of the fre-examples repo. Each model yaml can still point to it and it will be easier to maintain one yaml file versus multiple (that should really all be the same anyway).