Underline links with href in markdown content

What does this MR do?

Adds an underline to all <a href="..."> elements inside .md (markdown-rendered) content on design.gitlab.com.

Previously, only external links received an underline due to a separate rule, leaving internal links without one. This change ensures consistent underline styling for all links with an href attribute within markdown content, improving readability and accessibility.

The fix was identified in a discussion on !6278 (merged) and is a one-liner using the existing gl-underline Tailwind utility:

.md {
  a[href] {
    @apply gl-underline;
  }
}

Related items

• MR closes #2694

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Does this MR meet the acceptance criteria?

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Accessibility

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