Bug: mdx-loader caching issue => linked md files should be added as dependencies

## 🐛 Bug Report

Suppose you have:

```md
# md file

[link](./otherMdFile.md)
```

Then if the linked md file change from `slug: /path1` to `slug: /path2` the md file must be "invalidated"

Otherwise, we'll keep linking to `/path1` (instead of new `/path2`) due to Webpack caches.

mdx-loader probably needs to collect linked file paths, and call `this.addDependency(linkedFilePath);`

---

Issue noticed in PR https://github.com/facebook/docusaurus/pull/5755

https://app.netlify.com/sites/docusaurus-2/deploys/6171611d255a130008c35dff

Had to clear the Netlify cache to fix the build failure