Implement automated TOC generation from toc.md file

[Copilot]

This PR implements an automated Table of Contents (TOC) generation system that allows maintaining the mdBook SUMMARY.md file from a simpler toc.md format.

Problem

Previously, the book-src/SUMMARY.md file had to be manually maintained using mdBook's verbose format with specific indentation and linking patterns. This made TOC updates error-prone and time-consuming.

Solution

Created a simplified toc.md format and automated generation system:

Simple toc.md format:

## File System
- Read file line by line | 01-01-read-file-line-by-line.md
- Mmap file | 01-02-mmap-file.md
- Check file existence | 01-04-file-exists.md

## Cryptography  
- Calculate SHA-256 digest of a file | 02-01-sha-digest.md

Automatically generates mdBook's SUMMARY.md:

- [File System]()

  - [Read file line by line](./01-01-read-file-line-by-line.md)
  - [Mmap file](./01-02-mmap-file.md)
  - [Check file existence](./01-04-file-exists.md)

- [Cryptography]()

  - [Calculate SHA-256 digest of a file](./02-01-sha-digest.md)

Features

• Simplified Format: Clean, readable toc.md with section headers and pipe-separated items
• Exact Compatibility: Generated SUMMARY.md is byte-for-byte identical to the original
• Build Integration: Automatically runs during make serve, CI builds, and npm scripts
• Special Case Handling: Correctly handles Database section links, Text Processing formatting, and file path prefixes

Usage

# Manual generation
node generate-summary.js
make generate-summary
npm run generate-summary

# Automatic generation during:
make serve           # local development  
# CI/CD builds        # GitHub Actions

The toc.md file becomes the source of truth, while SUMMARY.md is auto-generated and should not be manually edited.

Fixes #107.

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