Add automated checks for broken links and Markdown formatting errors

[ankurdotb]

I'm sending this PR over as a suggestion to carry out automated broken link checks and flagging formatting errors in Markdown.

I've configured it so that:

1. Only new, modified, or edited files are alerted on - rather than all files (should be less annoying to contributors working on their own submission)
2. Doesn't stop in case it detects errors, but still shows the output
3. One common case of "broken" links was email address: I've added a configuration to ignore checking links for email addresses.
4. I've provided a default set of known good Markdown formatting rules, but these can be modified if necessary
5. Checks can be restricted to only certain folders if necessary

[elenachachkarova]

Thank you @ankurdotb for spending the time to help with this.