feat: show a warning message when md links are broken

[gianlucadonato]

🚀 Feature

Show a warning message in the console that outputs a log when an internal link inside the documentation is broken.

Fix #1111

Have you read the Contributing Guidelines on issues?

Yes.

Motivation

It can happen to have brokens links in the documentation that are caused by a change in the file id/path (e.g. a file is renamed, moved or simply there's a typo in the link) and then forgetting to update the link that points to it.
It's very hard discovering where they are in the docs and fix it.

Pitch

It would be nice to introduce a check (maybe flaggable in the options) when running docusaurus start or docusaurus build that shows a warning message if a link in the markdown has an unresolved path.

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[endiliey]

This needs more discussion, because technically some docusaurus user might actually want to link to some url with .md file extension.

@JoelMarcey @yangshun are u guys good with the proposed changes ?

It warns if any unresolved .md link

[endiliey]

Thanks @gianlucadonato 😄

[charpeni]

This needs more discussion, because technically some docusaurus user might actually want to link to some url with .md file extension.

This is correct, we link to .md files within React Native website and we now have a bunch of warnings during the build process.

Edit: Actually we have a bunch of warnings because versioned_docs is referencing some docs that do not exist anymore in docs, but they still exist in versioned_docs 🤔.