create ten (10) next beginner's topics

[geliettech]

What does this PR add or change?

This PR adds ten (10) beginner-friendly topics on Next.js, including:

• Introduction to Next.js
• Routing Mechanism in Next.js
• Custom Not Found (404) Pages in Next.js
• File Colocation in Next.js
• Private Folders in Next.js
• Understanding Layouts in Next.js
• Using Metadata in Next.js
• Template Files in Next.js
• Loading UI in Next.js
• Error Handling in Next.js

These articles are designed to help beginners understand the core concepts and features of Next.js through practical explanations and examples.

Type of contribution

• New article
• New case study
• New top-level topic (discussed in an issue first, includes folder README + first article)
• Improvement to existing content (fix, clarification, update)
• Repository / docs improvement

Checklist

• I read CONTRIBUTING.md
• My article is based on a template from templates/ and includes the metadata header (Topic, Level, Author)
• The file uses kebab-case naming and lives in the most relevant folder
• I added a link to my article in the folder's README.md index
• This is original content (not copied from another source)
• Content is practical and non-promotional