Adds empty state to model registry pages

What does this MR do and why?

Adds empty state to model registry pages

Adds empty state to models index and model details pages, linking to the docs.

Screenshots or screen recordings

Page	Before	After
Model index
Model detail

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:model_registry)
   Ml::FindOrCreateModelService.new(p, "model_1").execute

2. Navigate to <gdk>/<project>/-/ml/models. The empty state should be displayed, and clicking on the action button should navigate to the docs

3. Create a model

   p = Project.find_by(id: <your project id>)
   

4. Navigate to <gdk>/<project>/-/ml/models and select the model. The new empty state should display

MR acceptance checklist

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