Render challenge README.md instead of `od` field

[Berkanktk]

The challenge info page on admin.haaukins.dk currently renders the od field from challenge.yml as markdown.
However, the tutorials and solutions are usually much more detailed in each challenge’s README.md file within the repository.

If possible, it would be far more useful to pull and render the raw README.md from the GitLab repo instead of the od field.

Proposed solution:

• Fetch and display the challenge’s README.md content from the GitLab repository via API.
• Optionally fall back to the od field if the README.md is missing.