feat: add learning-path page with Cloud/Local sub-pages (EN/ZH/JA)

[lin-bot23]

Summary

Add a comprehensive Learning Path page for ComfyUI beginners, with dedicated Cloud and Local sub-pages. This addresses a long-standing gap where new users had no guided path through the docs.

Changes

New files (10 files, ~1359 lines added)

Main learning path (per language):

• get_started/learning-path.mdx
• zh/get_started/learning-path.mdx
• ja/get_started/learning-path.mdx

Cloud sub-page (per language):

• get_started/learning-path/cloud.mdx
• zh/get_started/learning-path/cloud.mdx
• ja/get_started/learning-path/cloud.mdx

Local sub-page (per language):

• get_started/learning-path/local.mdx
• zh/get_started/learning-path/local.mdx
• ja/get_started/learning-path/local.mdx

Modified files

• docs.json — registered learning-path in Get Started sidebar (after index, before install sections)

Content structure

Main page

1. What is ComfyUI? — Plain language explanation, why GPU is needed
2. Start with Templates — Recommended approach
3. Cloud vs Local — Comparison with RTX 6000 Pro and local model guidance
4. Custom Nodes — What they are, Cloud limitation (cannot install in shared cluster), native vs community comparison
5. Step-by-step learning path — Choose path → Interface → First Generation → Parameters → Tutorial sequence → Explore
6. Quick troubleshooting reference

Cloud sub-page

• Zero install, pre-installed models, RTX 6000 Pro
• Sign up → Pick template → Generate

Local sub-page

• Why GPU needed + requirements table
• Installation → Model download → First generation → Custom node guide

Design decisions

• Plain language, minimal jargon for beginners
• Cloud-first recommendation with clear local upgrade path
• Sub-pages use parent field for automatic breadcrumb navigation