YouTuber MrBeast's company emphasized the size of his audience and stunty videos in pitches to brands.Arturo Holmes - FIFA/FIFA via Getty ImagesNewly filed court documents reveal how MrBeast pitched brands and what his rates were in years past.They show how the YouTuber has become a marketing machine for brands.We broke down the takeaways of proposals for Slack, Sony, and Walmart.How much does it cost to advertise with MrBeast, the world's top Y…