Inaccesability of index when there also a file matching the directory name

[JJWTimmer]

Have you read the Contributing Guidelines on issues?

• [X] I have read the Contributing Guidelines on issues.

Prerequisites

• [X] I'm using the latest version of Docusaurus.
• [X] I have tried the npm run clear or yarn clear command.
• [X] I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• [X] I have tried creating a repro with https://new.docusaurus.io.
• [X] I have read the console error message carefully (if applicable).

Description

In e.g. /docs/module/ there could be an index.md and a module.mdx file. The module.mdx is served as root of the folder, and the index is unreachable, although it appears as 'next' link at the bottom of the module/ path. Clicking it stays on the same page though.

Reproducible demo

No response

Steps to reproduce

1. Create /doc/demo folder and in there:
2. Create a file index.md with a title like # Hello Index
3. Create a file demo.mdx with a title like # Hello Demo
4. Yarn start
5. Navigate to /docs/demo
6. Try to view both pages

Expected behavior

I expect there is a precedence, where index.md(x) is used as index and the demo.mdx is just a doc.

Actual behavior

/docs/demo will show you the content of demo.mdx, the content of index.mdx is ignored. Although the next-link shows index, you can't view it if you click it. It stays on the same page.

Your environment

• Public source code: https://github.com/usethesource/rascal-website
• Public site URL: eg. https://www.rascal-mpl.org/docs/Library/resource/jdbc/
• Docusaurus version used: 2.2.0
• Environment name and version (e.g. Chrome 89, Node.js 16.4):
• Operating system and version (e.g. Ubuntu 20.04.2 LTS):

Self-service

• [ ] I'd be willing to fix this bug myself.

[Josh-Cena]

Yes, I'm afraid we do the slug remapping too aggressively. To be clear: the issue is not about the sidebar category indexes, but slug clashing. /docs/module/index.md and /docs/module/module.md both map to the same slug /docs/module/. To get around it for the time being, add an explicit slug front matter for one of the files.

[Josh-Cena]

There are two ways to fix this: emit a warning/error telling the user to add an explicit front matter to one of the files, or have a priority of index > README > <category name>.