Revamped get started guide to be more similar to typical libraries

[chrisraygill]

Currently, our Get started guide uses genkit init to set up a new Genkit project from scratch, but I think this is too magical. We should show that it's extremely simple to add Genkit to your project. This version of the Get started guide is much more similar to that of the Gemini API, and more similar to typical library / API quickstart guides.

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