Add test to validate broken links in markdown files

[Sudan Landge]

We have lot of links in our md files which are broken and could be broken due to refactoring of code.
We need to add a new CI test which checked for all the links in all the md files in the repo.
The discussion started with #4341 (comment)
and we found mlc which can validate all links in the md files however it fails to validate links that are embedded like https://github.com/firecracker-microvm/firecracker/blob/main/docs/getting-started.md?plain=1#L79

The new test should cover links in all md files (embedded in code or not).

[Andrew Yao]

Hi everyone,

We're a group at UT Austin and we're interested in picking this issue up.