docs: add Japanese deep-dive guide for ymm4MCP explainers

[SCPgamerscp]

Motivation

• Provide a ready-to-use, long-form Japanese explainer that creators can reuse for解説動画やブログ, explaining what ymm4MCP is and how to present it.
• Surface the new guide from the project README so visitors can find video/blog guidance easily.

Description

• Add BLOG_VIDEO_GUIDE_JA.md, a 160+ line guide covering architecture, component responsibilities, production workflow, API usage, strengths, constraints, and narration templates for video/blog use.
• Update README.md to add a short “解説動画・ブログ向け詳細ガイド” section that links to BLOG_VIDEO_GUIDE_JA.md.
• Pure documentation change only; no runtime or API behavior was modified.

Testing

• Ran git diff --check to verify whitespace/format issues and it reported no problems.
• Ran python -m py_compile mcp-server/server.py as a sanity check on the Python server module and it compiled without errors.
• No automated unit tests were added because the change is documentation-only.

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Codex Task