feat: add hyperlink support in Markdown rendering for deb and snap pages

[deandreamatias]

Fix issue #1989.

• I created a custom widget generator for a attributes in Markdown. I could only use the onTapLink property of MarkdownBody, but that doesn't follow the project's design and accessibility guidelines.
• I'm not sure if creating the builder in packages/app_center/lib/widgets/hyperlink_text.dart is the correct place.
• I also needed to add the markdown package directly to pubspec. flutter_markdown already imports this package, but if don't import it directly, it will get depend_on_referenced_packages lint info.
• On video can see test with mouse pointer and also with keyboard navigation (tabs and space)

Screencast.From.2026-02-06.13-32-51.mp4