Add check_lint markdown target

[tylerauerbeck]

What this PR does / why we need it: This PR adds the check_lint markdown target to the Makefile in order lint all of the *.md files in the repository.

Does this PR fix any issue? _(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged)_:

Fixes #652

Special notes for your reviewer:

@mazzystr Tried to pick up some of the notes from #719. Figured this would be a good starting point. The current LINT_IMAGE is just a stand-in for now until https://github.com/igorshubovych/markdownlint-cli/pull/208 (hopefully) gets merged.

[kubevirt-bot]

Hi @tylerauerbeck. Thanks for your PR.

I'm waiting for a kubevirt member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[kubevirt-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: davidvossel

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