Add Check Markdown Link GH action

[MichaelSimons]

Related to #https://github.com/dotnet/dotnet-docker/issues/5316

These changes include:

• Add a GH action that validates links in all Markdown files. The action is the same one used in several dotnet repos already - core, monitor, etc.
• Fix the numerous invalid links surfaced by the new link validator.

[MichaelSimons]

I discovered comments are not permitted in the Check Markdown Link config file - https://github.com/gaurav-nelson/github-action-markdown-link-check/issues/201. This masked some additional broken links that I addressed in https://github.com/microsoft/dotnet-framework-docker/pull/1163/commits/1b389211158e583db3b7b764a41e091b1dc4d381.