Add testing for broken links

[Rafellus]

While making the translation for Spanish I found a broken link: Parity Demo-PoA Tutorial. I leave the link in the translation as the task wasn't about changing stuff. But we could use a Github Action to check for broken links on push to master.

[pi0neerpat]

I think thats a great idea. Is there some existing solution you had in mind?

[Rafellus]

I check some and after a check to the code, this is a great and simple option: https://github.com/lycheeverse/lychee-action. I'm testing the workflow in my fork, to send a pull request today, with all the config.

[Rafellus]

Found one that works. This: https://github.com/marketplace/actions/markup-link-checker-mlc

I ran Github Actions in my fork and got great results: https://github.com/Rafellus/ethereum-developer-tools-list/runs/2748708518?check_suite_focus=true. You can find there the workflow YAML file.

Also adding this will be a good opportunity for adding a status badge.

[sirhashalot]

This was fixed by PR #314