fix(import): RM-17139 Ensure no /docs/docs slugs

[xavierandueza]

🚥 Resolves RM-17139

🧰 Changes

• BEFORE: docs-site entry pages at generic routes could publish with generic guide slugs like docs.
• AFTER: generic docs route pages use a meaningful introduction slug.
• Generic docs categories no longer force docs-prefixed page slugs when cleaner slugs are available.

🧬 QA & Testing

• node --test src/commands/import.slug-planner.test.js
• npm test
• Runner smoke: npm run step:skeleton https://www.mintlify.com/docs
  • This was the original root issue
  • Now the /docs/docs -> /docs/getting-started-introduction
    • This isn't just "introduction" because there are 2x introduction now, one in we made from docs/index.md and one from docs/api/index.md
      • When 2x categories share the same final slug, we make them both keep rolling back until we have unique slugs.
    • Although this may seem a bit messy, its the best way to handle this in a consistent IMO

[xavierandueza]

Requesting changes: the slug normalization is collapsing terminal generic docs pages to the category slug in non-generic categories. That misses the main invariant and needs a small restructuring before merge.

[xavierandueza]

This combines the category segment with the URL segments before deciding whether a terminal generic docs route should fall back to introduction, which changes the invariant. A page at https://example.com/docs under a non-generic category currently becomes the category slug, not introduction:

const page = { title: 'Docs', url: 'https://example.com/docs' }
ensureUniqueSlugs([{ title: 'Guides', pages: [page] }]).get(page) // guides

That means the fix only works when the organizer happens to put the entry page under a generic category. The cleaner model is to normalize the URL/virtual path first, turn an all-generic path into ['introduction'], and only then add the category as the outermost expansion segment for collision disambiguation. That keeps the base slug behavior deterministic (introduction) while still allowing guides-introduction if it genuinely collides.

[xavierandueza]

(agent response)
Fixed — normalized the page URL/virtual path before adding the category expansion segment, so terminal generic docs pages resolve to introduction under non-generic categories while collisions can still expand to guides-introduction. Added regression coverage. 36108cb