Use a Markdown linter in PR validation

[lbussell]

We should add a test or pre-build validation step that runs a markdown linter to check for dead links and proper markdown formatting across all of our docs (not just generated readmes)

Related: #5297 and dotnet/docker-tools#1172

[richlander]

The team is trying to put my creative efforts in a straightjacket. "I cannot create under these conditions!"