Add documentation for Metal target

[swoods-nv]

Support for Metal should be documented as in prototype stage. Documentation must be added to docs/user-guide/09-targets.md to indicate that Metal is available as a work in progress (e.g. see note below CUDA and OptiX section). Additionally, it would be nice to document the format of the profile strings used by findProfile() in the appropriate places in docs/user-guide/08-compiling.md