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 https://github.com/dotnet/docker-tools/issues/1172

[richlander]

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

[MichaelSimons]

FYI, the markdownlint tool I have integrated into CI only validates intra document links that use the # anchor ID syntax. There are requests for markdownlint to support additional link checks. It appears a lot of project use https://github.com/tcort/markdown-link-check.

[MichaelSimons]

This work is now complete. The dotnet-docker and dotnet-framework have a markdown linter and markdown link validator GH action. All markdown issues within the repos have also been addressed