Add link validation workflow action.

[ThorntonMatthewD]

The workflow action is set to run whenever a push is made to the main branch, and it will also run every Monday at 9:15 AM. Feel free to adjust the time to your liking. Crontab.guru is a great tool if you're like me and don't remember cron notation off the top of your head. :laughing:

Drawbacks I've noticed:

• I've had to ignore Cloudflare links as their DDoS protection, even with a custom user agent, blocks the link validator's requests.
• Certain sites provoke a redirection loop. Currently it seems that https://developer.android.com/studio in /modules/tooling.md suffers from this issue, despite the site being accessible via a browser.
• If you have any syntax errors in your yaml or json files the workflow will run without any errors. However, instead of taking several minutes, it will only run for ~60 seconds before completing. That is the only consistent indication that I should check my syntax that I've found..

Overall it seems to work and in my tests it did catch a handful of dead links. I only wish it were more accurate.

Fixes #19

[ThorntonMatthewD]

Sorry, I didn't see #20 before opening this.