Add test to validate broken links in markdown files

[wearyzen]

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).

[andr3wy]

Hi everyone,

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

[roypat]

Hey all,
We added a test to ensure that inter-markdown links are not broken in #4421. We decided against adding something that also validates external links to our CI for multiple reasons:

1. We do not want to send a bunch of HTTP requests to random websites every time a PR gets updated
2. Events external to this repository could cause the test to fail (say a temporary outage of a linked website), causing such a test to fail intermittently
3. The mlc tool above has a lot of false positives (for example regarding images embedded in markdown).

For these reasons, we are closing this issue