docs: add docs for file imports

[kevinzunigacuellar]

Description

• Closes #250
• Adds docs for file imports using shortcuts file and solution

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[Nemikolh]

@AriPerkkio, I agree with your proposed structure! Long term we could even have the # Markdown support section describes everything that's possible but right now if it shows the most commonly used things and link to expressive-code that would be a great start :raised_hands:

[kevinzunigacuellar]

Thanks for the review and clarification. I wasn't sure if I should write an entire markdown/mdx section so I decided to start with just file imports.