feat(website): improve docs menu

[theosanderson]

Enhance DocsMenu Component with Improved Index Page Handling and Directory Sorting

Overview

This PR updates the DocsMenu component to better handle index pages and implement sorting of directories based on the order property of their respective index pages. These changes aim to improve the navigation structure and provide more control over the menu's organization.

Key Changes

1. Index Page Handling

   • Index pages (e.g., index.mdx) are now linked from the top-level directory names.
   • Index pages are excluded from the sublist of pages for each directory.

2. Directory Sorting

   • Directories are now sorted based on the order property of their index pages.

3. Directory Title Display

   • The displayed title for each directory now uses the title from the index page if available, falling back to the directory name if not.

4. Code Refactoring
   Less code duplication

[emmahodcroft]

I'm excited to have an improved docs menu but won't pretend I understand the changes! 😁