docs: improve guidelines quick start and add screenshots

[Cuzyoung]

Summary

• add standalone docs/guidelines.html and link it from docs/index.html
• merge installation and first-time quick start into one section
• reorder quick start flow: API config first, then paper input
• add quick start screenshots:
  • docs/assets/guidelines/quick_api.png
  • docs/assets/guidelines/quick_paper.png

Test Plan

• open docs/index.html and verify Guidelines entry works
• open docs/guidelines.html and verify quick start order and numbering
• verify both quick-start images render correctly