fix(docs): emit error on slug collision to prevent silent overwriting

[ananikets18]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes Inaccesability of index when there also a file matching the directory name #8555) and the maintainers have approved on my working plan.

Motivation

Fixes #8555

When a category directory contains both an index.md and a document matching the directory's name (e.g. module/index.md and module/module.mdx), they both map to the exact same slug (/docs/module/) via the isCategoryIndex logic. Previously, Docusaurus would silently allow this collision, leading to unpredictable routing where the index becomes unreachable.

Following the maintainer's suggestion on the issue, this PR implements a hard error that explicitly detects slug collisions. When a collision occurs, the build will now safely crash with a highly descriptive error guiding users to add a slug front matter to resolve the conflict.

Test Plan

Added a new unit test in loadVersion.test.ts with a mock fixture (version-with-slug-conflicts) that deliberately reproduces the demo/index.md and demo/demo.md clash.

Verified that compiling this fixture triggers the exact targeted error:

[Error: The docs plugin found docs sharing the same slug:

- url=/docs/demo/ found in number=2 docs:
  - versioned_docs/version-with-slug-conflicts/demo/demo.md
  - versioned_docs/version-with-slug-conflicts/demo/index.md

Docs should have distinct slugs.
In case of conflict, you can use the `slug` front matter to assign an explicit distinct slug to each doc.
    ]

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