Add Google AI Studio API key instructions to documentation

[xingyaoww]

This PR adds instructions on how to get a Google AI Studio API key to the documentation.

The instructions have been added to the "Getting an API Key" section in the Quick Start / Getting Started guide, next to the existing instructions for OpenAI and Anthropic.

Fixes #8937

@xingyaoww can click here to continue refining the PR

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:546a1b9-nikolaik   --name openhands-app-546a1b9   docker.all-hands.dev/all-hands-ai/openhands:546a1b9

[openhands-ai]

Looks like there are a few issues preventing this PR from being merged!

• GitHub Actions are failing:
  • Run Python Unit Tests

If you'd like me to help, just leave a comment, like

@OpenHands please fix the failing actions on PR #8938

Feel free to include any additional details that might help me get this PR into a better state.

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