Render Kramdown through Gitlab Markup

What does this MR do?

This MR changes the way Kramdown format is rendered. Instead of using Githup::Markup we're using Gitlab::Markup and avoiding some extensions that format has.

Besides, it's not a supported format in wikis, so makes sense to use the normal markdown pipeline.

Refs #324452 (closed)

Screenshots

This is how a Kramdown document is rendered now:

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content