Move documentation API code into a separate module

Simply moves documentation API from `Hex.API.Release` to `Hex.API.ReleaseDocs`, changing the method call names as such:

`API.Release.get/2` -> `API.ReleaseDocs.get/2`
`API.Release.new/5` -> `API.ReleaseDocs.new/5`
`API.Release.delete/3` -> `API.ReleaseDocs.delete/3`

Unit tests and mix tasks calling the old methods have been changed accordingly, and all pass.

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