Resolves #21
# Description
Issue: #21
- [x] Adds fully custom `react-markdown` integration
- [x] Demo display available at `/challenges/:any_number` ex https://mitxpro-dev.github.io/learnjs/challenges/1
- [x] Added a `Box` with `h={12}` below the navigation bar to push UI elements down past the navbar (since it is fixed, the navbar would overlap content)
# Screenshots
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# Notes
Styled components as I saw fit, but very open to making any improvements / changes :)
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