docs: improve quickstart guide and fix JSON examples

[jedrzejdocs-code]

Fixes #167

Summary

Improves documentation usability based on user feedback that the usage document is not user friendly.

Changes

• Add dedicated QUICKSTART.md for 5-minute setup
• Fix invalid JSON comments in configuration examples (JSON doesn't support // comments)
• Fix Docker localhost issue — use host.docker.internal with explanation
• Add Requirements table with Go 1.21+, Grafana 9.0+, Docker 20.10+
• Add config file paths per OS (macOS, Windows, Linux)
• Expand Troubleshooting section from 1 error to 10 common issues
• Add Example Queries section showing what users can ask
• Consolidate Environment Variables into single reference table
• Add inline service account creation steps instead of external link
• Reference /examples folder in docs