Make code blocks look cleaner

[buffalojoec]

Right now, code snippets and blocks are rendered on the docs site as dark black text with a white background. They should instead render as a more traditional Markdown code block, like this:

use nautilus::*;

#[nautilus]
mod program_nautilus {

    fn create_with_payer<'a>(
        mut new_wallet: Create<'a, Wallet<'a>>,
        rent_payer: Signer<Wallet<'a>>,
    ) -> ProgramResult {
        new_wallet.create_with_payer(rent_payer)
    }
}