docs: add documentation index and Spec Kit guide

Add comprehensive documentation index to README (EN + PT-BR):
- Getting Started section with quick links
- Architecture & Design docs
- Development guides
- Project management (roadmap, tasks)
- Package documentation

Create Spec Kit collaborative development guide:
- Complete 7-step workflow explanation
- Practical examples (file upload feature)
- How to contribute using Spec Kit
- Benefits for contributors
- FAQ section

Available in both English and Portuguese:
- docs/SPEC_KIT_GUIDE.md
- docs/pt-br/SPEC_KIT_GUIDE.md

This enables the community to contribute using structured,
collaborative development with clear specifications.