Integrate VSCode's Markdown Linter into CI

[carlos-zamora]

PR #4615 used the VS Code Markdown Linter to clean up our markdown a bit. It'd be nice if we could make this a part of our CI and just add it to the Invoke-CodeFormat tool in \tools\OpenConsole.psm1.

It looks like the PowerShell team is integrating at the project level, unsure about their CI integrations:
https://github.com/PowerShell/PowerShell/blob/master/.vscode/extensions.json#L10
https://github.com/PowerShell/PowerShell/blob/master/.vscode/settings.json#L10
Originally posted by @rpunt in #4615 (comment)

[DHowett-MSFT]

Yeah, might as well. Backlog, -triage.