docs: update Quick Start with clearer security warning and named steps

[clouatre]

Summary

Updates Quick Start section with clearer security messaging and descriptive step names.

Changes

• Alert type: Changed from [!WARNING] to [!IMPORTANT] (more appropriate severity)
• Security message: Clarified that prompt injection is a general AI risk, not specific to Goose or this action
• Step names: Added descriptive names to all steps (Checkout, Setup Goose CLI, Upload Analysis Artifact)
• Consistency: Matches pattern from setup-q-cli-action PR chore: bump actions/checkout from 4 to 5 #13

Why

• [!IMPORTANT] is more appropriate than [!WARNING] for guidance/best practices
• Expanded message helps users understand the risk applies to ANY AI tool
• Named steps improve workflow readability and debugging
• Example already uses safe pattern (tool output analysis)

Testing

• ✅ Verified markdown renders correctly
• ✅ All step names descriptive and consistent