Detect external broken links on build

🚀 Feature

Partly inspired by #3048, it would be nice to detect external links in the Markdown docs and signal to the user that they are broken.

Ref projects:

• https://github.com/tcort/markdown-link-check
• https://github.com/tcort/link-check

Have you read the Contributing Guidelines on issues?

Yes

Motivation

If you want good docs, you probably don’t want broken links.

Pitch

We should signal users if any external links are invalid on build.

A by-pass flag would be helpful because there might be cases broken external links are ok (e.g. pages that anticipate a link to become valid later on).

This additional check would be really helpful. I’m suggesting this feature because I am building a docs site on top of Docusaurus and I found a need for this personally. I was actually going to use markdown-link-checker with CI. But if this can be built-in, then it saves a LOT of work.

[anshulrgoyal]

Here is my pr for assets check https://github.com/facebook/docusaurus/pull/3096

[anshulrgoyal]

@adrianmcli it is very easy to create a new remark plugin for it. I have created one for image and one for assets u can take a look.