Add product research section and reorder sidebar

[disassembler]

Summary

• Adds a new Product Research section with vendor evaluation reports, a milestone template, and a submission guide
• Adds _category_.yml for product-research to control sidebar ordering
• Reorders sidebar: Vision → Product Research → Hardfork Planning → Workshops → Business Interviews

Test plan

• Verify sidebar order in local dev server
• Check all product research pages render correctly
• Confirm existing sections (Vision, Hardfork Planning, etc.) are unaffected