fix(docs:prune): prune child docs

[kanadgupta]

🚥 Fixes #710

🧰 Changes

Turns out our response payloads from GET /categories/{slug}/docs looks like this for pages that contain subpages:

[
  ...
  {
    "title": "Navigating Your Hub",
    "slug": "navigating-your-hub",
    "order": 4,
    "hidden": false,
    "children": [
      {
        "title": "Search + Search API",
        "slug": "search",
        "order": 0,
        "hidden": false
      },
      {
        "title": "Quick Nav in the API Reference",
        "slug": "quick-nav-api-reference",
        "order": 1,
        "hidden": false
      }
    ]
  }
]

This PR fixes our docs:prune command to account for child pages.

🧬 QA & Testing

Do tests pass?

[erunion]

api v2 can't come soon enough

[kanadgupta]

@erunion 💯