Implement Github action for dead url check

[mulles]

To avoid dead url's in the project one could use the following github action:
https://github.com/marketplace/actions/broken-link-check

Should we implement this in the CI for all documentation repos?

To check internal .md file links there are other methods:

https://github.com/ulivz/vuepress-plugin-check-md

vuejs/vuepress#1092

Should I add this to Github Projects ?

[martinjaeger]

Good idea!

I would not add it to the GitHub Project, because that's just for tracking the progress of chapters.

We can just keep this issue open until someone has added the check to the CI. I guess it will take a bit of effort to make it work properly.