Enable relative links in Docker image tags tables

[lbussell]

Context: dotnet/dotnet-docker#5872 (comment)

This allows specifying useRelativeLinks in the manifest per each readme, and if it's set, then the link to the Dockerfiles in the tags table for that readme will be a relative link instead of a GitHub permalink.

This is where you would specify this in the manifest: https://github.com/dotnet/dotnet-docker/blob/08e0e7c3345a75af15ca0c1733d4f07676e0997d/manifest.json#L21-L24

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[mthalman]

In what scenario would we not use relative links?

[lbussell]

In what scenario would we not use relative links?

Good point. I'll edit this to make them all use relative links. Should simplify the code some.

[lbussell]

I have updated the code to output relative links for readmes, but keep GitHub permalinks for MCR tags metadata, since we do push that metadata to MCR and they could decide to use those links again at any time. Also added tests.