Extend md-lint to ensure vignette() references are links for the website

[MichaelChirico]

          Yea, we can probably do something for _most_ cases with a simple regex (probably in the `md-lint` job) check for `vignette(..., package="data.table")` that's not part of a markdown link.

The case changed here for datatable-intro is much harder though, maybe an LLM could find it.

Originally posted by @MichaelChirico in #6617 (comment)

[venom1204]

Hi @MichaelChirico,
Is this issue still relevant? If so, I’d be happy to work on it. Could you please provide more details or guidance on how I should approach this?
Looking forward to your input!

[MichaelChirico]

yes, still needed!

I would go through the vignettes and find all cross-references vignettes (when one vignette links to another), and make sure that the website rendered version has a clickable link that resolved to the correct page.

Then come up with a rule enforceable by regular expression that can search through the vignettes and ensure this cross-linking continues to work going forward even as the vignettes change over time.