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.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[tonybaroneee]

I really dig this simplification. It's much more aligned with how developers are used to getting started with other node libraries. Thanks Chris!

[i2amsam]

Looks great!

[mjchristy]

Love it! And love the "what to do next section" at the bottom.