Add parallel intercepting routes in next

[geliettech]

What does this PR add or change?

Adds a new intermediate-level Next.js article explaining Parallel Routes, Intercepting Routes, and how to combine both patterns to build UI such as modal-based photo galleries. The article covers route conventions, slots, unmatched routes, conditional rendering, and practical 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