Add template-only markdown link linter.

[anybodys]

Problem

Links in markdown documentation can break as updates are made and it's difficult to track them all down to fix.

Solution

Add a linter that will check markdown files for broken links, along the lines of navapbc/template-infra#497

Since the infra template already had this linter, this template should use a "template-only" linter. That will work for this template but avoid having duplicate linters when the application template is used with the infra template.

Alternatives

Because this was already adopted in infra, we should use the same solution and not explore alternatives, unless we want to change all the templates.

Additional Context

navapbc/template-application-nextjs#264