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

### Have you read the Contributing Guidelines on issues?

- [X] I have read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/main/CONTRIBUTING.md#reporting-new-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.