[i8n-pt] Updating, improving and expanding current PT translation

[cdutr]

Summary

The Portuguese documentation has only 3 translated files out of 336 (0.9% coverage) and contains outdated
content and few grammar errors. This issue tracks fixing existing translations and adding critical getting-started
documentation.

Current status:

• index.md - grammar errors (line 21: verb agreement, adjective placement)
• installation.md - grammar errors (line 103: verb conjugation)
• quicktour.md - severely outdated (2023 version, uses SD 1.5 instead of modern models)

Comparison: Korean (54 files, 16%), Chinese (52 files, 15%), Portuguese (3 files, 0.9%)

Goals (This Issue)

Focus: Fix critical issues and add minimal core documentation

1. Fix grammar errors in existing 3 files
2. Update quicktour.md to 2025 version (match EN structure)
3. Add stable_diffusion.md (most requested guide)
4. Create in_translation.md placeholder
5. Update _toctree.yml with proper structure

Out of scope: Additional translations, video docs, community coordination (will be handled in follow-up
issues)

Translation Guidelines

Follow the TRANSLATING guide:

• Use informal tone (talking with a friend 🤗)
• Use gender-neutral language when possible
• Use Brazilian Portuguese (PT-BR) conventions
• Add translations to docs/source/pt/
• Use in_translation.md for untranslated pages (see docs/source/ko/ for reference)

Tasks

Critical Fixes

• Fix grammar in index.md (eg line 21: "queira" → "quer", "modular caixa" → "caixa modular", "Têm" → "Há")
• Fix grammar in installation.md (eg line 103: "irá" → "irão" or "vincular")
• Update quicktour.md to 2025 structure (use Qwen-Image, add modern examples)
• Translate stable_diffusion.md
• Create in_translation.md placeholder page
• Update _toctree.yml with in_translation markers

Next Steps (Future Issues)

After this issue is resolved, follow-up issues can address:

• Video generation pipeline documentation (HunyuanVideo, CogVideo, SVD, LTX)
• Additional tutorials and training guides
• Optimization guides (fp16, torch2.0, xformers)
• Community coordination for broader translation effort
• API reference translations

These are intentionally out of scope for this issue to keep it focused and achievable