chore(index): add jsdoc comments for static code analysis

[Fdawgs]

Added JSDoc comments for static code analysis and better IDE support (IntelliSense param hints and inline documentation in VS Code is a big bonus).

lib/dirList.js already has JSDoc comments as well, so this makes it consistent across the codebase!

Checklist

• run npm run test && npm run benchmark --if-present
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Copilot]

Pull Request Overview

This PR adds comprehensive JSDoc comments to the main index.js file to improve static code analysis and IDE support. The changes align with existing JSDoc practices in the codebase and enhance developer experience through better IntelliSense and documentation.

Key Changes

• Added JSDoc type annotations for function parameters and return types
• Included proper type imports for Fastify types and project-specific types
• Enhanced inline documentation for complex function signatures

Comments suppressed due to low confidence (1)

index.js:500

• The findIndexFile function is missing JSDoc documentation. Add JSDoc comments with parameter types and return type for consistency with other functions in the file.

function findIndexFile (pathname, root, indexFiles = ['index.html']) {

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Uzlopak]

LGTM