Simplify book structure from 4 parts to 3 parts for better organization

[Copilot]

This PR addresses feedback that the book's comprehensive topic coverage needed a simpler overall structure. The changes reorganize the Programming Best Practices book from a 4-part structure to a cleaner 3-part structure while maintaining all content.

Problem Addressed

The original 4-part structure had cognitive complexity issues:

• Uneven distribution of chapters across parts (3-3-3-1)
• The single-chapter "Part IV" felt disconnected
• Related practical content was scattered across multiple parts
• Navigation was less intuitive for readers

Solution Implemented

Before (4 parts):

第I部：基礎原則編（第1-3章）
第II部：設計原則編（第4-6章）  
第III部：実践技法編（第7-9章）
第IV部：現代的技術編（第10章）

After (3 parts):

第I部：プログラミングの基本原則（第1-3章）
第II部：設計と開発の実践（第4-8章）
第III部：チームと現代的実践（第9-10章）

Key Benefits

• Cognitive simplification: 3-part structure follows the "rule of 3" for better comprehension
• Balanced distribution: More even chapter allocation (3-5-2 vs 3-3-3-1)
• Logical flow: Natural progression from individual fundamentals → design/development practices → team/modern approaches
• Unified practical content: Chapters 4-8 now form a cohesive design-to-implementation journey
• Better navigation: Cleaner structure for readers to follow their learning path

Changes Made

• Updated table-of-contents.md with new 3-part organization
• Updated README.md to reflect simplified structure throughout
• Updated planning documents (計画と進捗.md) with new organization
• Created comprehensive documentation of the simplification process
• Maintained all existing content without any loss

The book now provides the same comprehensive coverage of programming best practices with a much more approachable and intuitive structure.

Fixes #101.

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[Copilot]

Pull Request Overview

This PR simplifies the book structure from a 4-part organization to a cleaner 3-part structure to improve cognitive accessibility and navigation. The restructuring addresses feedback about uneven chapter distribution and scattered practical content across multiple parts.

Key changes include:

• Reorganizing from 4 parts (3-3-3-1 chapter distribution) to 3 parts (3-5-2 distribution)
• Consolidating related design and development content into a unified Part II
• Moving from "基礎原則編/設計原則編/実践技法編/現代的技術編" to "プログラミングの基本原則/設計と開発の実践/チームと現代的実践"

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
計画と進捗/計画と進捗.md	Updates project planning document with new 3-part structure
構成シンプル化提案.md	New documentation detailing the simplification proposal and rationale
構成シンプル化実施結果.md	New documentation recording implementation results and achieved improvements
website/MarkDown/table-of-contents.md	Restructures main table of contents with new 3-part organization
README.md	Updates project overview to reflect simplified structure throughout