Add simple lightbox feature to markdown images

What does this MR do?

• Open full-screen view of image in a lightbox when clicked

Why was this MR needed?

• Improve the UX of viewing images in MRs and Issues

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #28387 (closed)