feat(mdx-loader): Remark plugin to report unused MDX / Markdown anchors links

[OzakIOne]

Pre-flight checklist

• [x] I have read the Contributing Guidelines on pull requests.
• [x] 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 #0000) and the maintainers have approved on my working plan.

Motivation

As of today, Docusaurus only check broken links and numerous people would like Docusaurus to check broken anchors.

I've made a PoC remark plugin that aims to detect broken anchors and warn the user about it, so far here is the functionality implemented :

• [x] Detect broken anchors in the same file
• [ ] Detect broken anchors in an other file
• [ ] Detect broken anchors of headings having a custom id # Title {#custom-id}
• [ ] Support mdx files with react pages

For the custom id it should be fairly easy to support it, the problem is just that in the jest tests the remark plugin for custom id doesn't seems to work, and implement the custom id in the rendered html, it just renders it as text

For the broken anchors related to an other .md file, I don't know how to implement it, I thought we could do it in this remark plugin by recursively checking .md files and checking the anchors however it doesn't seems to be the right way to do it.

For now the only way for users to check the anchors in other files is to setup a ci with remark validate links

Test Plan

Test links

yarn jest brokenAnchors --watch

Deploy preview: https://deploy-preview-9512--docusaurus-2.netlify.app/

Related issues/PRs

https://github.com/facebook/docusaurus/issues/3321

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