feat(docs): add flexible multi-level directory support #6

We propose adding a flexible multi-level directory structure to improve content organization and navigation for the documentation site.

Proposed Solution

Folder-as-a-Book approach, widely adopted by top documentation sites such as Vercel, Next.js, and Stripe Docs.
Core idea: use the file system hierarchy to reflect the logical structure of the docs.

• Each topic/series = a folder
• Overview page = index.mdx inside the folder
• Child articles = other .mdx files within the folder
• Use numeric prefixes (01-, 02-) to define default ordering

Example Structure

content/
└── computer-science/
    └── data-structures/
        ├── 01-introduction.mdx       (standalone article)
        ├── array/                    (series "Array")
        │   ├── index.mdx             (overview)
        │   ├── 01-static-array.mdx
        │   └── 02-dynamic-array.mdx
        └── linked-list/              (series "Linked List")
            ├── index.mdx             (overview)
            ├── 01-singly-linked-list.mdx
            └── 02-doubly-linked-list.mdx

Benefits

• Highly intuitive: anyone opening the repo can instantly understand content structure.
• Clean, semantic URLs: automatically generate /data-structures/linked-list and /data-structures/linked-list/singly-linked-list.
• Easy maintenance: related content lives together; adding, deleting, or editing is straightforward.
• Automatic navigation: sidebar navigation can be generated recursively, no manual config needed.