docs: update GenUI references in Get Started guide

[victoreronmosele]

This PR updates the GenUI references in the Get Started guide.

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[gemini-code-assist]

Code Review

This pull request updates the documentation in 'get-started.md' to align with recent API changes in the 'genui' and 'firebase_ai' packages, including updated dependency names, simplified model and chat message initializations, and revised widget builder signatures. The review feedback correctly points out that the 'highlightLines' parameter on a code block needs to be updated to reflect the shifted lines.

[gemini-code-assist]

The highlightLines parameter is still set to 4-5, but because of the addition of systemPromptFragments: [ on line 784, the lines to highlight have shifted. It should be updated to 5-6 to correctly highlight the system instruction lines.

[victoreronmosele]

Agreed. Updating.

[victoreronmosele]

Done.