Broken Markdown link detection should also work for internal links

[Zamiell]

One fantastic feature of Docusaurus 2 is that it warns me if I have a broken link in one of my Markdown files:

warn Docs markdown link couldn't be resolved: (non_existent_page.md) in C:\Repositories\my-repo\docs\main.md for version current

This feature is really helpful and a huge time saver!

However, this feature does not work with internal links on the page.

For example, consider the following Markdown file:

### Section 1

This is section 1. By the way, you might want to also check out [section 2](#section-3).
<!-- Oops, I made a typo at the end of the previous line, and the link is now broken! -->

### Section 2

This is section 2.

Docusaurus won't warn me about the bug in this file, which is a real bummer. =(
Is this something that can be added?

[slorber]

Yes, we definitively want this feature, duplicate of #3321