[Issue 2275] Create external-sorting-algorithms.md

[daaimah123]

📝 Description

This PR adds a comprehensive topic outline on External Sorting Algorithms to the curriculum. The outline provides both theoretical knowledge and practical implementation guidance. The content includes detailed explanations of various external sorting techniques, their applications, and self-guided activities for hands-on learning.

🔂 Changes Made

• Added a new topic outline on External Sorting Algorithms in the algorithms folder
• Created detailed explanations of external sorting concepts including External Merge Sort, Polyphase Merge Sort, Replacement Selection, and External Quicksort
• Developed four self-guided activities with complete instructions and starter code
• Included JavaScript implementations and examples throughout the content
• Added a section on common mistakes and misconceptions
• Ensured all activities are self-contained without external links

⚙️ Related Issue

Issue Number: #2275

🍏 Type of Change

New Topic Outline

🎁 Acceptance Criteria

• Content is appropriate for career changers transitioning to full stack engineering at a remedial to intermediate level
• Topic outline includes all required sections (prerequisites, objectives, motivation, specific things to learn, activities, common mistakes)
• Activities are self-guided and do not require external resources
• Code examples use JavaScript and are well-explained
• Real-world applications and examples are included

🧪 How to test or evaluate

1. Review the topic outline for completeness and accuracy of external sorting concepts
2. Check that the activities can be completed independently without external resources
3. Test the provided JavaScript code snippets to ensure they work as expected
4. Confirm that real-world applications and examples help learners understand the practical value of external sorting algorithms

[daaimah123]

merging these in while supporting contributors for the ODHack14 also because over a month stale