Improve README hero for developer conversion

[httpdss]

Summary

• Refresh the README hero with a sharper YAML-first / AI-agent positioning statement.
• Add a 60-second quickstart, Docker equivalent, audience section, early comparison table, and clearer CTAs.
• Keep the existing demo GIF visible with more descriptive alt text.

Verification

• structkit list | head -40
• structkit generate --structures-path structkit/contribs --var module_name=my-terraform-module terraform/modules/generic /tmp/structkit-readme-verify --file-strategy overwrite
• README local link check via Python script
• git diff --check

Closes #153