Add documentation Markdown linter to CI

[baentsch]

As per original suggestion by @beldmit (#1784 (comment)).

[praveksharma]

I suggest we use a maintained tool (such as markdownlint; including this as an example not suggesting we use specifically this tool) instead of maintaining our own script.

Since a linter would target specific markdown spec(s) I suggest we also choose which spec we want out markdown files to conform to (in addition to GH flavoured markdown). This may also ease future discussion/resolution for doc generation issues.

[baentsch]

I suggest we use a maintained tool

Fully agree. Instead of starting a discussion on a spec and as we're really only talking about GH documents, what about "just" using the GHdocs linter setup?

[planetf1]

I'd agree with @baentsch that if the target is github markdown then using the github-supported configuration makes sense.

I use markdownlint (defaults, in vscode) myself as a first-pass checker.