Add CEP 5 : Adopting Grayskull as the defacto conda recipe generator

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Added draft CEP for adopting Grayskull as the de facto conda recipe generator.

Abstract conda-skeleton, the conda recipe generator, unfortunately, is not well maintained. It is also deeply integrated with conda build itself. Grayskull, a community innovation, on the other hand is a stand alone conda recipe generator. Migrating away from conda-skeleton and adopting Grayskull as the de facto conda recipe generator would allow easy maintenance and faster generation of recipes.