Add Cortex deployment guide

[caleb-kaiser]

Add a guide for deploying fastai models with Cortex:

https://github.com/cortexlabs/cortex

[jph00]

Thanks @caleb-kaiser ! Could you please add some info about recommended instance types, scaling params, or anything else the user might need to consider, and provide some pricing estimates for a few common options? For most users, they'll likely be most interested in what the minimum cost would be to host a simple model (e.g rn34 image classifier) on a CPU instance to show off to their friends and family.

[jph00]

Oh and add it to the sidebar, at the end of the "production" section: https://github.com/fastai/course-v3/blob/master/docs/_data/sidebars/home_sidebar.yml