Improve contribution templates and guidelines for better PR descriptions

[Copilot]

This PR addresses the poor quality contribution snippets that were causing confusion and incomplete PR descriptions, as evidenced by issues #134, #135, #144, #150, and #152.

Problem

The existing contribution templates were causing several issues:

• Vague guidance: The PR template had unhelpful "EDIT ME" placeholders that contributors weren't replacing
• Missing examples: No clear examples of what constitutes a good vs poor description
• Outdated references: Links to Hacktoberfest 2019 that were no longer relevant
• Lack of structure: No guidance on technical details like complexity analysis or testing

This resulted in PRs with descriptions like:

• "Please include a small description of your algorithm/data structure and add reference to the Hacktoberfest issue (#hacktoberfest2018 #66)" (template text not replaced)
• "Hey! I added XYZ Algorithm." (too brief, no technical details)

Solution

Enhanced PULL_REQUEST_TEMPLATE.md:

• Replaced generic placeholders with structured sections and checkboxes
• Added clear examples in comments showing good description format
• Included sections for implementation details (programming language, time/space complexity)
• Added testing verification checklist

Improved CONTRIBUTING.md:

• Added comprehensive "Writing a Good Pull Request Description" section
• Provided concrete examples of good vs poor descriptions based on actual problematic PRs
• Added code quality standards and documentation requirements
• Fixed grammar issues and made references more timeless

Updated README.md:

• Removed outdated 2019 Hacktoberfest references
• Added "What Makes a Great Contribution" section with clear quality standards
• Improved structure and navigation to contribution guidelines

Impact

Contributors now have:

• Clear, actionable guidance instead of confusing placeholders
• Concrete examples of quality descriptions
• Structured templates that prompt for all necessary information
• Better understanding of technical requirements and quality standards

These changes address the root cause of the poor PR descriptions seen in the referenced issues by providing educational, structured guidance that helps contributors write better submissions from the start.

Fixes #155.

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