Document refund policy

### Problem

We don't currently have our refund policy documented anywhere. Stripe recommends linking it in Checkout to increase conversions.

### Proposal

We should probably find a boilerplate refund policy and add it to the website to make it more clear to customers

### Prior Art (Optional)

I think Printful might have a stock refund policy for store items that we could use and then clarify the difference for the download