Support for symbolic linked .md files

### 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#issues).

### Motivation

This issue is kind of a continuation from:

- https://github.com/facebook/docusaurus/issues/3579
- And https://github.com/facebook/docusaurus/issues/3272#issuecomment-2778515784

I have a repository with many helm charts, docker images, and whatnot. I like to keep a README.md for each of these little sub-projects, right inside each folder.

That's so when people open the directories in GitHub, they will have the README rendered.

And also, I'd like to show them in my documentation portal.

For now, I'm creating symbolic links to all these READMEs into my `website/docs` dir, so that they get parsed by Docusaurus too. For that to work, I need to use the WebPack `resolve.symlinks: false` trick.

And it works great, except it does not work when the Docusaurus project is installed with `pnpm install` (and also `deno install`, which works the same way as pnpm).

So, it would be great if Docusaurus had direct support for reading symlinked `.md` files.

### Self-service

- [ ] I'd be willing to do some initial work on this proposal myself.