docs: Improve CoreConcepts.md structure with '5 Core Principles'

[takeshishimada]

Summary

• Restructured CoreConcepts.md to clearly communicate Flow's design philosophy through "The 5 Core Principles"
• Enhanced explanations with better benefits and rationale for each principle
• Aligned formatting with other documentation files

Key Improvements

Structure

• ✅ Organized as "The 5 Core Principles" with clear numbering and hierarchy
• ✅ Each principle now has explicit Benefits sections
• ✅ Moved middleware to "Additional Features" (not a core principle)

New Content

• ✅ Principle 2: View-Local State: Aligned with SwiftUI Philosophy

  • Explains why Flow avoids global stores
  • Contrasts with other libraries that create store hierarchies
  • Emphasizes alignment with SwiftUI's philosophy

• ✅ Enhanced Principle 4: MainActor Isolation

  • Improved benefits list:
    • Code locality - all related code in one place
    • Intuitive - write naturally like regular Swift
    • Compile-time safety - data races caught at compile time
    • No manual dispatch needed
  • Added comparison with traditional patterns

Formatting

• ✅ Removed separator lines (---) to match other docs (GettingStarted, PracticalGuide)
• ✅ Consistent heading structure throughout

Inspiration

This structure is inspired by the Medium article's "Flowの5つのコア原則" section, which clearly communicates Flow's design philosophy. The English documentation now has the same clarity and persuasiveness.

Test Plan

• Review the five principles for clarity and accuracy
• Verify formatting consistency with other documentation files
• Check that all code examples are correct
• Ensure all doc links work properly

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