docs: add quick start guide for Gemini CLI

docs/quick-start.md

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+# Quick Start Guide
+
+Get Gemini CLI running in your GitHub repository in under 10 minutes.
+
+## Prerequisites
+
+- A GitHub repository you control
+- One of: Gemini API key, Google Cloud project, or GitHub App
+
+## Option 1: Gemini API Key (Fastest - 5 minutes)
+
+**Step 1: Get a free API key**
+1. Visit https://aistudio.google.com/app/apikey
+2. Click "Create API Key"
+3. Copy the key
+
+**Step 2: Add to GitHub**
+1. Go to your repository → Settings → Secrets and variables → Actions
+2. Click "New repository secret"
+3. Name: `GEMINI_API_KEY`
+4. Paste your API key
+5. Save
+
+**Step 3: Create a workflow file**
+Create `.github/workflows/gemini-basic.yml`:
+
+```yaml
+name: Gemini Review
+
+on:
+  pull_request:
+    types: [opened, synchronize]
+
+jobs:
+  review:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: write
+    steps:
+      - uses: actions/checkout@v4
+      - uses: google-github-actions/run-gemini-cli@v0
+        with:
+          gemini_api_key: ${{ secrets.GEMINI_API_KEY }}
+          prompt: "Review this pull request for code quality and potential issues."
+```
+
+**Done!** Open a PR to test it.
+
+---
+
+## Option 2: Automated Setup in CLI
+
+```bash
+# Install Gemini CLI locally
+gemini
+
+# In the CLI terminal, run:
+/setup-github
+
+# Follow prompts to configure GitHub workflows
+```
+
+---
+
+## Option 3: Google Cloud / Vertex AI (Most Secure)
+
+Run the setup script:
+
+```bash
+./scripts/setup_workload_identity.sh \
+  --repo "owner/your-repo" \
+  --project "your-gcp-project"
+```
+
+The script handles all configuration automatically.
+
+---
+
+## Next Steps
+
+### Trigger Gemini in Comments
+
+Once workflows are set up, mention Gemini in issues or PRs:
+
+```
+@gemini-cli explain this function
+
+@gemini-cli /review check for security issues
+
+@gemini-cli suggest improvements
+```
+
+### Customize Your Workflow
+
+Edit the workflow file to change:
+
+- **Prompt**: What task Gemini should perform
+- **Triggers**: When it runs (PR opened, schedule, etc.)
+- **Permissions**: What access level it needs
+
+### Explore Pre-Built Workflows
+
+Check [examples/workflows](../examples/workflows/README.md) for:
+- PR review automation
+- Issue triage automation
+- Scheduled code analysis
+- General-purpose assistant
+
+---
+
+## Troubleshooting
+
+**API Key not working?**
+- Verify it's added to repository secrets (not variables)
+- Check it's named exactly `GEMINI_API_KEY`
+- Test the key in Google AI Studio
+
+**Workflow doesn't run?**
+- Check repository has `.github/workflows/` directory
+- Verify the workflow syntax is valid (lint it in VS Code)
+- Check Actions tab for error messages
+
+**Need help?**
+- See detailed docs in [docs/](../docs/) folder
+- Check [CONTRIBUTING.md](../CONTRIBUTING.md) for development info
+- Open an issue on GitHub
+
+---
+
+## What's Next?
+
+- Customize prompts for your team's needs
+- Set up multiple workflows for different tasks
+- Create a [GEMINI.md](https://github.com/google-gemini/gemini-cli/blob/main/docs/get-started/configuration.md#context-files-hierarchical-instructional-context) with project guidelines
+- Explore [extensions](../docs/extensions.md) for custom capabilities